AIRPLANE 
PHOTOGRAPHY 

HERBERT  E.IVES 


AIRPLANE  PHOTOGRAPHY 


AIRPLANE 
PHOTOGRAPHY 


BY 

HERBERT  E.  IVES 

ft  f* 

MAJOR,  AVIATION  SECTION,  SIGNAL  OFFICERS  RESERVE  CORPS,  UNITED  STATES 
ARMY;  LATELY  OFFICER  IN  CHARGE  OF  EXPERIMENTAL  DEPART- 
MENT, PHOTOGRAPHIC  BRANCH,  AIR  SERVICE 


208  ILLUSTRATIONS 


PHILADELPHIA  AND  LONDON 
J.  B.  LIPPINCOTT  COMPANY 


COPTBIGHT,    1920,    BT   i,  B.   LIPPINCOTT  COMPANY 


PRINTED   BY  J.    B.   LIPPINCOTT   COMPANY 

AT  THE  WASHINGTON  SQUARE  PRESS 

PHILADELPHIA,  XT.  8.  A. 


TO  MY  WIFE 

A  HELPFUL  CRITIC,  EVEN  THOUGH  SHE 
NEITHER    PHOTOGRAPHS   NOR    FLIES 


435217 


PREFACE 

AIRPLANE  photography  had  its  birth,  and  passed  through 
a  period  of  feverish  development,  in  the  Great  War.  Prob- 
ably to  many  minds  it  figures  as  a  purely  military  activity. 
Such  need  not  be  the  case,  for  the  application  of  aerial 
photography  to  mapping  and  other  peace-time  problems 
promises  soon  to  quite  overshadow  its  military  origin.  It 
has  therefore  been  the  writer's  endeavor  to  treat  the  sub- 
ject as  far  as  possible  as  a  problem  of  scientific  photog- 
raphy, emphasizing  those  general  principles  which  will 
apply  no  matter  what  may  be  the  purpose  of  making  photo- 
graphs from  the  air.  It  is  of  course  inevitable  that  who- 
ever at  the  present  time  attempts  a  treatise  on  this  newest 
kind  of  photography  must  draw  much  of  his  material  from 
war-time  experience.  If,  for  this  reason,  the  problems  and 
illustrations  of  this  book  are  predominantly  military,  it 
may  be  remembered  that  the  demands  of  war  are  far  more 
severe  than  those  of  peace;  and  hence  the  presumption  is 
that  an  account  of  how  photography  has  been  made  suc- 
cessful in  the  military  plane  will  serve  as  an  excellent  guide 
to  meeting  the  peace-time  problems  of  the  near  future. 

It  is  assumed  that  the  reader  is  already  fairly  conversant 
with  ordinary  photography.  Considerable  space  has  indeed 
been  devoted  to  a  discussion  of  the  fundamentals  of  photog- 
raphy, and  to  scientific  methods  of  study,  test,  and  speci- 
fication. This  has  been  done  because  aerial  photography 
strains  to  the  utmost  the  capacity  of  the  photographic 
process,  and  it  is  necessary  that  the  most  advanced  methods 
be  understood  by  those  who  would  secure  the  best  results 
or  contribute  to  future  progress.  No  pretence  is  made  that 

7 


8  PREFACE 

the  boot:  is  an  aerial  photographic  encyclopaedia;  it  is  not  a 
manual  of  instructions;  nor  is  its  appeal  so  popular  as  it 
would  be  were  the  majority  of  the  illustrations  striking 
aerial  photographs  of  war  subjects.  It  is  hoped  that  the 
middle  course  steered  has  produced  a  volume  which  will  be 
informative  and  inspirational  to  those  who  are  seriously 
interested  either  in  the  practice  of  aerial  photography  or  in 
its  development. 

The  writer  is  deeply  in  debt  to  many  people,  whose 
assistance  of  one  sort  or  another  has  made  this  book  possible. 
First  of  all  should  be  mentioned  those  officers  of  the  English, 
French  and  Italian  armies  through  whose  courtesy  it  is 
that  he  can  speak  at  first  hand  of  the  photographic  prac- 
tices in  these  armies  at  the  front.  It  is  due  to  Lieutenant 
Colonel  R.  A.  Millikan  that  the  experimental  work  of  which 
the  writer  has  had  charge  was  initiated  in  the  United  States 
Air  Service.  To  him  and  to  Major  C.  E.  Mendenhall,  under 
whom  the  work  was  organized  in  the  Science  and  Research 
Division  of  the  Signal  Corps,  are  owing  the  writer's  thanks 
for  the  opportunities  and  support  given  by  them.  A  similar 
acknowledgment  is  made  to  Lieutenant  Colonel  J.  S.  Sullivan, 
Chief  of  the  Photographic  Branch  of  the  Army  Air  Service,  for 
his  interest  and  encouragement  in  the  compilation  of  this 
work,  and  for  the  permission  accorded  to  use  the  air  service 
photographs  and  drawings  which  form  the  majority  of 
the  illustrations. 

The  greatest  debt  of  all,  however,  is  to  those  officers  who 
have  formed  the  staff  of  the  Experimental  Department.  To 
mention  them  by  name:  Captain  C.  A.  Proctor,  who  was 
charged  with  our  foreign  liaison,  and  who  acted  as  deputy 
chief  during  the  writer's  absence  overseas;  Captain  A.  K. 
Chapman,  in  charge  of  the  work  on  optical  parts,  and  later 
chief  of  our  Rochester  Branch;  Captain  E.  F.  Kingsbury, 


PREFACE  9 

who  had  immediate  charge  of  camera  development;  Lieu- 
tenant J.  B.  Brinsmade  and  Mr.  R.  P.  Went  worth,  who 
handled  the  experimental  work  on  camera  mountings  and 
installation;  Lieutenant  A.  H.  Nietz,  in  charge  of  the 
Langley  Field  Laboratory  of  the  Experimental  Department; 
Mr.  R.  B.  Wilsey  and  Lieutenant  J.  M.  Hammond,  who,  with 
Lieutenant  Nietz,  carried  on  the  experimental  work  on 
sensitized  materials.  A  large  part  of  what  is  new  and  what  is 
ascribed  in  the  following  chapters  to  "The  American  Air 
Servce  "  is  the  work  of  this  group  of  men.  Were  individual 
references  made,  in  place  of  this  general  and  inclusive  one, 
their  names  would  thickly  sprinkle  these  pages.  It  has  been 
a  rare  privilege  to  have  associates  so  able,  enthusiastic, 
and  loyal. 

THE  AUTHOR 

NOVEMBER,  1919 


CONTENTS 

I.  INTRODUCTORY 


CHAPTER  PAGE 

1.  GENERAL  SURVEY 15 

2.  THE  AIRPLANE  CONSIDERED  AS  A  CAMERA  PLATFORM 20 

II.  THE  AIRPLANE  CAMERA 

3.  THE  CAMERA — GENERAL  CONSIDERATIONS 39 

4.  LENSES  FOR  AERIAL  PHOTOGRAPHY '  44 

5.  THE  SHUTTER 68 

6.  PLATE-HOLDERS  AND  MAGAZINES 87 

7.  HAND-HELD  CAMERAS  FOR  AERIAL  WORK  95 

8.  NON-AUTOMATIC  AERIAL  PLATE  CAMERAS 102 

9.  SEMI-AUTOMATIC  AERIAL  PLATE  CAMERAS 116 

10.  AUTOMATIC  AERIAL  PLATE  CAMERAS 124 

11.  AERIAL  FILM  CAMERAS 130 

12.  MOTIVE  POWER  FOR  AERIAL  CAMERAS 145 

13.  CAMERA  AUXILIARIES 163 

III.  THE  SUSPENSION  AND  INSTALLATION  OF  AIRPLANE   CAMERAS 

14.  THEORY  AND  EXPERIMENTAL  STUDY  OF  METHODS  OF  CAMERA  SUSPENSION  .   179 

15.  PRACTICAL  CAMERA  MOUNTINGS 193 

16.  INSTALLATION  OF  CAMERAS  AND  MOUNTINGS  IN  PLANES 208 

IV.  SENSITIZED  MATERIALS  AND  CHEMICALS 

17.  THE  DISTRIBUTION  OF  LIGHT,  SHADE  AND  COLOR  IN  THE  AERIAL  VIEW *2?1 

18.  CHARACTERISTICS  OF  PHOTOGRAPHIC  EMULSIONS 227 

19.  FILTERS 239 

20.  EXPOSURE  OF  AERIAL  NEGATIVES 247 

21.  PRINTING  MEDIA 252 

22.  PHOTOGRAPHIC  CHEMICALS 257 

V.  METHODS  OF  HANDLING  PLATES,  FILMS  AND  PAPERS 

23.  THE  DEVELOPING  AND  DRYING  OF  PLATES  AND  FILMS 267 

24.  PRINTING  AND  ENLARGING.  . , 279 


K  CONTENTS 

VI.  PRACTICAL  PROBLEMS  AND  DATA 

25.  SPOTTING 291 

26.  MAP  MAKING 304 

27.  OBLIQUE  AERIAL  PHOTOGRAPHY 320 

28.  STEREOSCOPIC  AERIAL  PHOTOGRAPHY 329 

29.  THE  INTERPRETATION  OF  AERIAL  PHOTOGRAPHS 351 

30.  NAVAL  AERIAL  PHOTOGRAPHY 368 

VII.  THE  FUTURE  OF  AERIAL  PHOTOGRAPHY 

31.  FUTURE  DEVELOPMENTS  IN  APPARATUS  AND  METHODS 383 

32.  TECHNICAL  AND  PICTORIAL  USES 388 

33.  EXPLORATION  AND  MAPPING.  .  401 


I 

INTRODUCTORY 


AIRPLANE  PHOTOGRAPHY 

CHAPTER  I 
GENERAL  SURVEY 

Aerial  Photography  from  Balloons  and  Kites. — Photog- 
raphy from  the  air  had  been  developed  and  used  to  a  limited 
extent  before  the  Great  War,  but  with  very  few  exceptions 
the  work  was  done  from  kites,  from  balloons,  and  from 
dirigibles.  Aerial  photographs  of  European  cities  had 
figured  to  a  small  extent  in  the  illustration  of  guidebooks, 
and  some  aerial  photographic  maps  of  cities  had  been  made, 
notably  by  the  Italian  dirigible  balloon  service.  Kites  had 
been  employed  with  success  to  carry  cameras  for  photo- 
graphing such  objects  as  active  volcanoes,  whose  phenomena 
could  be  observed  with  unique  advantage  from  the  air, 
and  whose  location  was  usually  far  from  balloon  or 
dirigible  facilities. 

As  a  result  of  this  pre-war  work  we  had  achieved  some 
knowledge  of  real  scientific  value  as  to  photographic  condi- 
tions from  the  air.  Notable  among  these  discoveries  was 
the  existence  of  a  veil  of  haze  over  the  landscape  when  seen 
from  high  altitudes,  and  the  consequent  need  for  sensitive 
emulsions  of  considerable  contrast,  and  for  color-sensitive 
plates  to  be  used  with  color  filters. 

The  development  of  aerial  photography  would  probably 
however  have  advanced  but  little  had  it  depended  merely 
on  the  balloon  or  the  kite.  As  camera  carriers  their  limita- 
tions are  serious.  The  kite  and  the  captive  balloon  cannot 
navigate  from  place  to  place  in  such  a  way  as  to  permit  the 

15 


16          AIRPLANE  PHOTOGRAPHY 

rapid  or  continuous  photography  of  extended  areas.  The 
kite  suffers  because  the  camera  it  supports  must  be  manipu- 
lated either  from  the  ground  or  else  by  some  elaborate 
mechanism,  both  for  pointing  and  for  handling  the  exposing 
and  plate  changing  devices.  The  free  balloon  is  at  the 
mercy  of  the  winds  both  as  to  its  direction  and  its  speed  of 
travel.  The  dirigible  balloon,,  as  it  now  exists  after  its 
development  during  the  war,  is,  it  is  true,  not  subject  to  the 
shortcomings  just  mentioned.  Indeed,  in  many  ways  it  is 
perhaps  superior  to  the  airplane  for  photographic  purposes, 
since  it  affords  more  space  for  camera  and  accessories,  and 
is  freer  from  vibration.  It  is  capable  also  of  much  slower 
motion,  and  can  travel  with  less  danger  over  forests  arid 
inaccessible  areas  where  engine  failure  would  force  a  plane 
down  to  probable  disaster.  But  the  smaller  types  as  at 
present  built  are  not  designed  to  fly  so  high  as  the  airplane, 
and  the  dirigibles,  both  large  and  small,  are  far  more  ex- 
pensive in  space  and  maintenance  than  the  plane.  For  this 
one  reason  especially  they  are  not  likely  to  be  the  most 
used  camera  carriers  of  the  aerial  photographer  of  the  future. 
Inasmuch  as  the  photographic  problems  of  the  plane  are 
more  difficult  than  those  of  the  dirigible  and  at  the  same  time 
broader,  the  subject  matter  of  this  book  applies  with  equal 
force  to  photographic  procedure  for  dirigibles. 

Development  of  Airplane  Photography  in  the  Great  War. 
—The  airplane  has  totally  changed  the  nature  of  warfare. 
It  has  almost  eliminated  the  element  of  surprise,  by  render- 
ing impossible  that  secrecy  which  formerly  protected  the 
accumulation  of  stores,  or  the  gathering  of  forces  for  the 
attack,  a  flanking  movement  or  a  "strategic  retreat."  To 
the  side  having  command  of  the  air  the  plans  and  activities 
of  the  enemy  are  an  open  book.  It  is  true  that  more  is 
heard  of  combats  between  planes  than  of  the  routine  task 


GENERAL  SURVEY  17 

of  collecting  information,  and  the  public  mind  is  more  apt 
to  be  impressed  by  the  fighting  and  bombing  aspects  of 
aerial  warfare.  Nevertheless,  the  fact  remains  that  the  chief 
use  of  the  airplane  in  war  is  reconnaissance.  The  airplane  is 
"the  eye  of  the  army." 

In  the  early  days  of  the  war,  observation  was  visual.  It 
was  the  task  of  the  observer  in  the  plane  to  sketch  the  out- 
lines of  trenches,  to  count  the  vehicles  in  a  transport  train, 
to  estimate  the  numbers  of  marching  men,  to  record  the 
guns  in  an  artillery  emplacement  and  to  form  an  idea  of 
their  size.  But  the  capacity  of  the  eye  for  including  and 
studying  all  the  objects  in  a  large  area,  particularly  when 
moving  at  high  speed,  was  soon  found  to  be  quite  too  small 
to  properly  utilize  the  time  and  opportunities  available  in 
the  air.  Moreover,  the  constant  watching  of  the  sky  for  the 
"Hun  in  the  sun"  distracted  the  observer  time  and  time 
again  from  attention  to  the  earth  below.  Very  early  in  the 
war,  therefore,  men's  minds  turned  to  photography.  The 
all-seeing  and  recording  eye  of  the  camera  took  the  place 
of  the  observer  in  every  kind  of  work  except  artillery  fire 
control  and  similar  problems  which  require  immediate 
communication  between  plane  and  earth. 

The  volume  of  work  done  by  the  photographic  sections 
of  the  military  air  service  steadily  increased  until  toward 
the  end  of  the  war  it  became  truly  enormous.  The  aerial 
negatives  made  per  month  in  the  British  service  alone 
mounted  into  the  scores  of  thousands,  and  the  prints  dis- 
tributed in  the  same  period  numbered  in  the  neighborhood 
of  a  million.  The  task  of  interpreting  aerial  photographs 
became  a  highly  specialized  study.  An  entirely  new  activity 
—that  of  making  photographic  mosaic  maps  and  of  main- 
taining them  correct  from  day  to  day — usurped  first  place 
among  topographic  problems.  By  the  close  of  the  war 


18          AIRPLANE  PHOTOGRAPHY 

scarcely  a  single  military  operation  was  undertaken  without 
the  preliminary  of  aerial  photographic  information.  Photog- 
raphy was  depended  on  to  discover  the  objectives  for 
artillery  and  bombing,  and  to  record  the  results  of  the  sub- 
sequent "shoots "  and  bomb  explosions.  The  exact  configu- 
rations of  front,  second,  third  line  and  communicating 
trenches,  the  machine  gun  and  mortar  positions,  the  "pill 
boxes,"  the  organized  shell  holes,  the  listening  posts,  and 
the  barbed  wire,  were  all  revealed,  studied  and  attacked 
entirely  on  the  evidence  of  the  airplane  camera.  Toward  the 
end  of  the  war  important  troop  movements  were  possible 
only  under  the  cover  of  darkness,  while  the  development 
of  high  intensity  flashlights  threatened  to  expose  even 
these  to  pitiless  publicity. 

Limitations  to  Airplane  Photography  Set  by  War  Con= 
ditions. — The  ability  of  the  pilot  to  take  the  modern  high- 
powered  plane  over  any  chosen  point  at  any  desired  altitude 
in  almost  any  condition  of  wind  or  weather  gives  to  the  plane 
an  essential  advantage  over  the  photographic  kites  and 
balloons  of  pre-war  days.  There  are,  however,  certain  dis- 
advantages in  the  use  of  the  plane  which  must  be  overcome 
in  the  design  of  the  photographic  apparatus  and  in  the 
method  of  its  use.  Some  few  of  these  disadvantages  are 
inherent  in  the  plane  itself ;  for  instance,  the  necessity  for 
high  speed  in  order  to  remain  in  the  air,  and  the  vibration 
due  to  the  constantly  running  engine.  Others  are  peculiar 
to  war  conditions,  and  their  elimination  in  planes  for  peace- 
time photography  will  give  great  opportunities  for  the 
development  of  aerial  photography  as  a  science. 

Chief  among  the  war-time  limitations  is  that  of  economy 
of  space  and  weight.  A  war  plane  must  carry  a  certain 
equipment  of  guns,  radio-telegraphic  apparatus  and  other 
instruments,  all  of  which  must  be  readily  accessible.  Many 


GENERAL  SURVEY  19 

planes  have  duplicate  controls  in  the  rear  cockpit  to  enable 
the  observer  to  bring  the  plane  to  earth  in  case  of  accident 
to  the  pilot.  Armament  and  controls  demand  space  which 
must  be  subtracted  from  quarters  already  cramped,  so  that 
in  most  designs  of  planes  the  photographic  outfit  must  be 
accommodated  in  locations  and  spaces  wretchedly  inade- 
quate for  it.  Economy  in  weight  is  pushed  to  the  last  ex- 
treme, for  every  ounce  saved  means  increased  ceiling  and 
radius  of  action,  a  greater  bombing  load,  more  ammunition, 
or  fuel  for  a  longer  flight.  Hence  comes  the  constant  pres- 
sure to  limit  the  weight  of  photographic  and  other  apparatus, 
even  though  the  tasks  required  of  the  camera  constantly  call 
for  larger  rather  than  smaller  equipment. 

To  another  military  necessity  is  due  in  great  measure 
the  forced  development  of  aerial  photographic  apparatus 
in  the  direction  of  automatic  operation.  The  practice  of 
entrusting  the  actual  taking  of  the  pictures  to  observers 
with  no  photographic  knowledge,  whose  function  was 
merely  to  "press  the  button"  at  the  proper  time,  necessi- 
tated cameras  as  simple  in  operation  as  possible.  The 
multiplicity  of  tasks  assigned  to  the  observer,  and  in  particu- 
lar the  ever  vital  one  of  watching  for  enemy  aircraft,  made 
the  development  of  largely  or  wholly  automatic  cameras 
the  war-time  ideal  of  all  aerial  photographic  services. 
Whether  the  freeing  of  the  observer  from  other  tasks  will 
relegate  the  necessarily  complex  and  expensive  automatic 
camera  to  strictly  military  use  remains  to  be  seen. 


CHAPTER  II 

THE  AIRPLANE  CONSIDERED  AS  A  CAMERA 
PLATFORM 

AN  essential  part  of  the  equipment  of  either  the  aerial 
photographer  or  the  designer  of  aerial  photographic  appara- 
tus is  a  working  knowledge  of  the  principles  and  construction 
of  the  airplane,  and  considerable  actual  experience  in  the  air. 


FIG.  1.— The  elements  of  the  plane. 

Conditions  and  requirements  in  the  flying  plane  are  far 
different  from  those  of  the  shop  bench  or  photographic 
studio.  As  a  preliminary  to  undertaking  any  work  on  air- 
plane instruments  a  good  text-book  on  the  principles  of 
flight  should  be  studied.  Such  general  ideas  as  are  necessary 
for  understanding  the  purly  photographic  problems  are, 
however,  outlined  in  the  next  paragraphs. 
20 


CAMERA  PLATFORM  21 

Construction  of  the  Airplane. — The  modern  airplane 
(Fig.  1)  consists  of  one  or  more  planes,  much  longer  across 
than  in  the  direction  of  flight  (aspect  ratio).  These  are 
inclined  slightly  upward  toward  the  direction  of  travel,  and 
their  rapid  motion  through  the  air,  due  to  the  pull  of  the 
propeller  driven  by  the  motor,  causes  them  to  rise  from  the 
earth,  carrying  the  fuselage  or  body  of  the  airplane.  In  the 
fuselage  are  carried  the  pilot,  observer,  and  any  other  load. 
Wheels  below  the  fuselage  forming  the  under-carriage  or 
landing  gear  serve  to  support  the  body  when  running  along 
the  ground  in  taking  off  or  landing.  The  pilot,  sitting  in 
one  of  the  cockpits,  has  in  front  of  him  the  controls,  by  means 
of  which  the  motion  of  the  plane  is  guided  (Figs.  2  and  3). 
These  consist  of  the  engine  controls — the  contacts  for  the 
ignition,  the  throttle,  the  oil  and  gasoline  supply,  air  pressure, 
etc.,  and  the  steering  controls — the  rudder  bar,  the  stick 
and  the  stabilizer  control.  The  rudder  bar,  operated  by  the 
feet,  controls  both  the  rudder  of  the  plane,  which  turns  the 
plane  to  right  or  left  in  the  air,  and  the  tail  skid,  for  steering 
on  the  ground.  The  stick  is  a  vertical  column  in  front  of  the 
pilot  which,  when  pushed  forward .  or  back,  depresses  or 
raises  the  elevator  and  makes  the  machine  dive  or  climb. 
Thrown  to  either  side  it  operates  the  ailerons  or  wing  tips, 
which  cause  the  plane  to  roll  about  its  fore  and  aft  axis. 
The  stabilizer  control  is  usually  a  wheel  at  the  side  of  the 
cockpit,  whose  turning  varies  the  angle  of  incidence  of  the 
small  stabilizing  plane  in  front  of  the  elevator,  to  correct 
the  balance  of  the  plane  under  different  conditions  of  loading. 

The  fuselage  consists  usually  of  a  light  hollow  framework 
of  spruce  or  ash,  divided  into  a  series  of  bays  or  compart- 
ments by  upright  members,  connecting  the  longerons,  which 
are  the  four  corner  members,  running  fore  and  aft,  of  the 
plane.  Diagonally  across  the  sides  and  faces  of  these  bays 


AIRPLANE  PHOTOGRAPHY 


CAMERA  PLATFORM  23 

are  stretched  taut  piano  wires,  and  the  whole  structure  is 
covered  with  canvas  or  linen  frabic.  Cross- wires  and  fabric 
are  omitted  from  the  top  of  one  or  more  bays  to  permit  their 
being  used  as  cockpits  for  pilot  and  observer.  In  later  designs 
of  planes  the  wire  and  fabric  construction  has  been  super- 
seded by  ply-wood  veneer,  thereby  strengthening  the  fuse- 
lage so  that  many  of  the  diagonal  bracing  wires  on  the  inside 


FIG.  4. — Biplane  in  flight. 

are  dispensed  with.  This  greatly  increases  the  accessibility 
of  the  spaces  in  which  cameras  and  other  apparatus  must 
be  carried. 

The  fuselage  differs  greatly  in  cross-section  shape  and  in 
roominess  according  to  the  type  of  engine.  In  the  majority 
of  English  and  American  planes,  with  their  vertical  cylinder 
or  V  type  engines,  the  fuselage  is  narrow  and  rectangular 
in  cross-section.  In  many  French  planes,  radial  or  rotary 
engines  are  used  and  the  fuselage  is  correspondingly  almost 
circular,  and  so  is  much  more  spacious  than  the  English 


24          AIRPLANE  PHOTOGRAPHY 

and  American  planes  of  similar  power.  The  shape  and  size 
of  the  plane  body  has  an  important  bearing  on  the  question 
of  camera  installation. 

Types  of  Planes. — The  most  common  type  of  plane  is 
the  biplane  (Fig.  4) ,  with  its  two  planes,  connected  by  struts 
and  wires,  set  not  directly  over  each  other,  but  staggered, 
usually  with  the  upper  plane  leading.  Monoplanes  were  in 
favor  in  the  early  days  of  aviation,  and  triplanes  have  been 


Fio.  5. — A  single-seater. 


used  to  some  extent.  According  to  the  position  of  the 
propeller  planes  are  classified  as  tractors  or  pushers,  tractors 
being  at  present  the  more  common  form.  Planes  are  further 
classified  as  single-seaters  (Fig.  5), two-seaters,  and  three-seaters . 
These  motor  and  passenger  methods  of  classification  are 
now  proving  inadequate  with  the  rapid  development  of 
planes  carrying  two,  three,  and  even  more  motors,  divided 
between  pusher  and  tractor  operation,  and  carrying  increas- 
ingly large  numbers  of  passengers.  Aside  from  structure, 
planes  may  be  further  classified  according  to  their  uses,  as 


CAMERA  PLATFORM  25 

scout,  combat,  reconnaissance,  bombing,  etc.  Planes  equipped 
with  floats  or  pontoons  for  alighting  on  the  water  are  called 
seaplanes  (Fig.  182),  and  those  in  which  the  fuselage  is  boat- 
shaped,  to  permit  of  floating  directly  on  the  water,  are 
flying  boats  (Fig.  183). 

The  Plane  in  the  Air. — The  first  flight  of  the  photographic 
observer  or  of  the  instrument  expert  who  is  to  work  upon 
airplane  instruments  is  very  profitably  made  as  a  "joy  ride," 
to  familiarize  him  with  conditions  in  the  air.  His  experience 
will  be  somewhat  as  follows: 

The  plane  is  brought  out  of  the  hangar,  carefully  gone 
over  by  the  mechanics,  and  the  engine  "warmed  up."  The 
pilot  minutely  inspects  all  parts  of  the  "ship,"  then  climbs 
up  into  the  front  cockpit.  He  wears  helmet  and  goggles, 
and  if  the  weather  is  cold  or  if  he  expects  to  fly  high,  a  heavy 
wool-lined  coat  or  suit,  with  thick  gloves  and  moccasins,  or 
an  electrically  heated  suit.  The  passenger,  likewise  attired, 
climbs  into  the  rear  cockpit  and  straps  himself  into  the  seat. 
He  finds  himself  sitting  rather  low  down,  with  the  sides  of 
the  cockpit  nearly  on  a  level  with  his  eyes.  To  either  side 
of  his  knees  and  feet  are  taut  wires  leading  from  the  controls 
to  the  elevator,  stabilizer,  tail  skid  and  rudder.  If  the 
machine  is  dual  control,  the  stick  is  between  his  knees,  the 
rudder  bar  before  his  feet.  None  of  these  must  he  let  his 
body  touch,  so  in  the  ordinary  two-seater  his  quarters  are 
badly  cramped. 

At  the  word  "contact"  the  mechanics  swing  the  pro- 
peller, and,  sometimes  only  after  several  trials,  the  motor 
starts,  with  a  roar  and  a  rush  of  wind  in  the  passenger's  face. 
After  a  moment's  slow  running  it  is  speeded  up,  the  inter- 
mittent roar  becomes  a  continuous  note,  the  plane  shakes 
and  strains,  while  the  mechanics  hold  down  the  tail  to 
prevent  a  premature  take-off.  When  the  engine  is  properly 


26          AIRPLANE  PHOTOGRAPHY 

warmed  up  it  is  throttled  to  a  low  speed,  the  chocks  under 
the  wheels  are  removed,  the  mechanics  hold  one  end  of  the 
lower  wing  so  that  the  plane  swings  around  toward  the  field. 
It  then  "taxis"  out  to  a  favorable  position  facing  into  the 
wind  with  a  clear  stretch  of  field  before  it.  After  a  careful 
look  around  to  see  that  no  other  planes  are  landing,  taking 
off,  or  in  the  air  near  by,  the  pilot  opens  out  the  engine,  the 
roar  increases  its  pitch,  the  plane  moves  slowrly  along  the 
ground,  gathers  speed  and  rises  smoothly  into  the  air.  Near 
the  ground  the  air  is  apt  to  be  "bumpy,"  the  plane  may 
drop  or  rise  abruptly,  or  tilt  to  either  side.  The  pilot  in- 
stantly corrects  these  deviations,  and  the  plane  continues  to 
climb  until  steadier  air  is  reached. 

At  first  the  passenger's  chief  impressions  are  apt  to  be  the 
deafening  noise  of  the  motor,  the  heavy  vibration,  the 
terrific  wind  in  his  face.  If  he  raises  his  hand  above  the 
edge  of  the  cockpit  he  realizes  the  magnitude  of  wind  resist- 
ance at  the  speed  of  the  plane,  and  hence  the  importance 
of  the  stream-line  section  of  all  struts  and  projecting  parts. 

When  he  reaches  the  desired  altitude  the  pilot  levels  off 
the  plane  and  ceases  to  climb.  Now  his  task  is  to  maintain 
the  plane  on  an  even  keel  by  means  of  the  controls,  correct- 
ing as  soon  as  he  notes  it,  any  tendency  to  "pitch," 
to  "roll"  or  to  "yaw"  off  the  course.  The  resultant  path 
is  one  which  approximates  to  level  straight  flying  to  a 
degree  conditioned  by  the  steadiness  of  the  air  and  the  skill 
of  the  pilot.  If  he  is  not  skilful  or  quick  in  his  reactions  he 
may  keep  the  plane  on  its  level  course  only  by  alternately 
climbing  and  gliding,  by  flying  with  first  one  wing  down  and 
then  the  other,  by  pointing  to  the  right  and  then  to  the  left. 
The  skilled  photographic  pilot  will  hold  a  plane  level  in 
both  directions  to  within  a  few  degrees,  but  he  will  do  this 
easily  only  if  the  plane  is  properly  balanced.  If  the  load 


CAMERA  PLATFORM  27 

on  the  plane  is  such  as  to  move  the  center  of  gravity  too  far 
forward  with  respect  to  the  center  of  lift  the  plane  will  be  nose- 
heavy,  if  the  load  is  too  far  back  it  will  be  tail-heavy.  Either 
of  these  conditions  can  be  corrected,  at  some  cost  in  efficiency, 
by  changing  the  inclination  of  the  stabilizer.  When  the  plane 
reaches  high  altitudes  in  rare  air,  where  it  can  go  no  further, 
it  is  said  to  have  reached  its  ceiling.  It  here  travels  level 
only  by  pointing  its  wings  upward  in  the  climbing  position, 
so  that  the  fuselage  is  no  longer  parallel  to  the  direction  of 
flight.  An  understanding  of  these  pecularities  of  the  plane 
in  flight  is  of  prime  importance  in  photographic  map  making, 
where  the  camera  should  be  accurately  vertical  at  all  times. 

The  direction  and  velocity  of  the  wind  must  be  carefully 
considered  by  the  pilot  in  making  any  predetermined  course 
or  objective.  The  progress  of  the  plane  due  to  the  pull  of 
the  propeller  is  primarily  with  reference  to  the  air.  If  this 
is  in  motion  the  plane's  ground  speed  and  direction  will  be 
altered  accordingly.  In  flying  with  or  against  the  wind  the 
ground  speed  is  the  sum  or  difference,  respectively,  of  the 
plane's  air  speed  (determined  by  an  air  speed  indicator)  and 
the  speed  of  the  wind.  If  the  predetermined  course  lies 
more  or  less  across  the  wind  the  plane  must  be  pointed  into 
the  wind,  in  which  case  its  travel,  with  respect  to  the  earth, 
is  not  in  the  line  of  its  fore  and  aft  axis.  The  effect  of  "crab- 
bing, "  as  it  is  called,  on  photographic  calculations  is  discussed 
later  (Figs.  136  and  138). 

When  the  plane  has  reached  the  end  of  its  straight 
course  and  starts  to  turn,  its  level  position  is  for  the  moment 
entirely  given  up  in  the  operation  of  banking  (Fig.  6). 
Just  as  the  tracks  on  the  curve  of  a  railroad  are  raised  on 
the  outer  side  to  oppose  the  tendency  of  the  train  to  slip 
outward,  so  the  plane  must  be  tilted,  by  means  of  the 
ailerons,  toward  the  inside  of  the  turn.  A  point  to  be 


28          AIRPLANE  PHOTOGRAPHY 

clearly  kept  in  mind  about  a  bank  is  that  if  correctly  made 
a  plumb   line  inside  the  fuselage   will   continue  to  hang 


FIG.  6.— Banking. 


vertical  with  respect  to  the  floor  of  the  plane,  and  not  with 
respect  to  the  earth,  for  the  force  acting  on  it  is  the  combina- 


CAMERA  PLATFORM  29 

i 
/ 

tion  of  gravity  and  the  acceleration  outward  due  to  the  turn, 
pnly  some  form  of  gyroscopically  controlled  pointer,  keep- 
ing its  direction  in  space,  will  indicate  the  inclination  of  the 
plane  with  respect  to  the  true  vertical.  If  the  banking  is 
insufficient  the  plane  will  side  slip  outward  or  skid;  if  too 
great,  it  will  side  slip  inward. 

As  part  of  the  "  joy  ride  "  the  pilot  may  do  a  few  "stunts," 
such  as  a  "stall,"  a  "loop,"  a  "  tail  spin,"  or  an  "Immel- 
man."  From  the  photographic  standpoint  these  are  of 
interest  in  so  far  as  they  bear  on  the  question  of  holding  the 
camera  in  place  in  the  plane.  The  thing  to  be  noted  here  is 
that  (particularly  in  the  loop),  if  these  maneuvers  are 
properly  performed,  there  is  little  tendency  toward  relative 
motion  between  plane  and  apparatus.  In  a  perfect  loop  it 
would,  for  instance,  be  unnecessary,  due  to  the  centrifugal 
force  outward,  for  the  observer  to  strap  himself  in.  It  is, 
however,  unwise  to  place  implicit  confidence  in  the  perfection 
of  the  pilot's  aerial  gymnastics.  No  apparatus  should  be 
left  entirely  free,  although,  for  the  reason  given,  compara- 
tively light  fastenings  are  usually  sufficient. 

When  nearing  the  landing  field  the  pilot  will  throttle 
down  the  engine  and  commence  to  glide.  If  he  is  at  a  con- 
siderable altitude  he  may  come  down  a  large  part  of  the  dis- 
tance in  a  rapid  spiral.  As  the  earth  is  approached  the  air 
pressure  increases  rapidly,  and  the  passenger,  if  correctly 
instructed,  will  open  his  mouth  and  swallow  frequently  to 
equalize  the  air  pressure  on  his  ear  drums.  Just  before  the 
ground  is  reached  the  plane  is  leveled  off,  it  loses  speed,  and, 
if  the  landing  is  perfect,  touches  and  runs  along  the  ground 
without  bouncing  or  bumping.  Frequently,  however,  the 
impact  of  the  tail  is  sufficiently  hard  to  cause  it  to  bump 
badly,  with  a  consequent  considerable  danger  to  apparatus 
of  any  weight  or  delicacy.  This  is  especially  apt  to  occur 


30          AIRPLANE  PHOTOGRAPHY 

in  hastily  chosen  and  poorly  leveled  fields  such  as  must  often 
be  utilized  in  war  or  in  cross-country  flying. 

Appearance  of  the  Earth  from  the  Plane. — The  view 
from  the  ordinary  two-seater  is  greatly  restricted  by  the 
engine  hi  front  and  by  the  planes  to  either  side  and  below 
(Figs.  7,  8,  and  9).  By  craning  his  neck  over  the  side,  or 
by  looking  down  through  an  opening  in  the  floor,  the  passen- 
ger has  an  opportunity  to  learn  the  general  appearance  of 
the  subject  he  is  later  to  devote  his  attention  to  photograph- 
ing. Perhaps  the  most  striking  impression  he  receives  will 
be  that  of  the  flatness  of  the  earth,  both  in  the  sense  of 
absence  of  relief  and  in  the  sense  of  absence  of  extremes  of 
light  and  shade.  The  absence  of  relief  is  due  to  the  fact 
that  at  ordinary  flying  heights  the  elevations  of  natural 
objects  are  too  small  for  the  natural  separation  of  the  eyes 
to  give  any  stereoscopic  effect.  The  absence  of  extremes  of 
light  and  shade  is  in  part  due  to  the  fact  that  the  natural 
surfaces  of  earth,  grass  and  forest  present  no  great  range  of 
brightness;  in  part  to  the  small  relative  areas  of  the  parts 
in  shadow;  in  considerable  part  to  the  layer  of  atmospheric 
haze  which  lies  as  an  illuminated  veil  between  the  observer 
and  the  earth  at  altitudes  of  2000  meters  and  over  (Figs. 
10  and  11).  Due  to  the  combination  of  these  factors  the 
earth  below  presents  the  appearance  of  a  delicate  pastel. 

As  the  gaze  is  directed  away  from  the  territory  directly 
below,  the  thickness  of  atmosphere  to  be  pierced  rapidly 
increases,  until  toward  the  horizon  (which  lies  level  with 
the  observer  here  as  on  the  ground)  all  detail  is  apt  to  be 
obliterated  to  such  an  extent,  that  only  on  very  clear  days 
can  the  horizon  itself  be  definitely  found  or  be  distinguished 
from  low  lying  haze  or  clouds  (Fig.  4). 

Airplane  Instruments. — Mounted  on  boards  in  front  of 
the  pilot  and  observer  are  various  instruments  to  indicate 


CAMERA  PLATFORM  31 


i 


FIG.  8.— The  view  astern. 


AIRPLANE  PHOTOGRAPHY 


CAMERA  PLATFORM 


33 


the  performance  of  engine  and  plane  (Fig.  2).  Those  of 
interest  to  the  photographic  observer  are  the  compass,  the 
altimeter,  the  air  speed  indicator,  the  inclinometers. 

The  compass  is  usually  a  special  airplane  compass,  with 
its  "card"  immersed  in  a  damping  liquid.  Like  most  of  the 
direction  indicating  instruments  on  a  plane  its  indications 


YIG.  10. — Appearance  of  tt 


are  only  of  significance  when  the  plane  is  pursuing  a  steady 
course.  On  turns  or  rapid  changes  of  direction  of  any  sort 
perturbations  prevent  accurate  reading. 

The  altimeter  is  of  the  common  aneroid  barometer  type. 
On  American  instruments  it  is  usually  graduated  to  read  in 
100-foot  steps.  While  somewhat  sluggish,  it  is  quite  satis- 
factory for  all  ordinary  determinations  of  altitude  in  photo- 


34          AIRPLANE  PHOTOGRAPHY 

graphic  work.  Were  primary  map  making  to  be  undertaken, 
where  the  scale  was  determinable  only  from  the  altitude  and 
focal  length  of  the  lens,  the  ordinary  altimeter  is  hardly 
accurate  enough. 

The  air  speed    indicator  consists   of  a  combination  of 
Venturi  and  Pitot  tubes,  producing  a  difference  of  pressure 


FIG.  11. — Appearance  of  the  earth  from  a  high  altitude — 10,000  feet  or  more. 

when  in  motion  through  the  air  which  is  measured  on  a 
scale  calibrated  in  air  speed.  This  instrument  is  important 
for  determining,  in  combination  with  wind  speed,  the 
ground  speed  of  the  plane,  on  the  basis  of  which  is  calcu- 
lated the  interval  between  exposures  to  secure  overlapping 
photographs.  Its  accuracy  is  well  above  that  necessary 
for  the  purpose. 


CAMERA  PLATFORM  35 

Inclinometers  for  showing  the  lateral  and  fore  and  aft 
angle  of  the  plane  with  the  horizontal,  are  occasionally  used, 
and  have  also  been  incorporated  in  cameras.  The  important 
point  to  remember  about  these  instruments  is  that  they  are 
controlled  not  alone  by  gravity  but  as  well  by  the  acceler- 
ation of  the  plane  in  any  direction.  They  consequently 
indicate  correctly  only  when  the  plane  is  flying  straight. 
On  a  bank  the  lateral  indicator  continues  to  indicate  "ver- 
tical" if  the  bank  is  properly  calculated  for  the  turn. 


II 

THE  AIRPLANE  CAMERA 


CHAPTER  III 
THE  CAMERA— GENERAL  CONSIDERATIONS 

Chief  Uses  of  an  Airplane  Camera. — The  kinds  of 
camera  suitable  for  airplane  use  and  the  manner  in  which 
they  must  differ  from  cameras  for  use  on.  the  ground  are 
determined  by  consideration  of  the  nature  of  the  work  they 
must  do.  Four  kinds  of  pictures  constitute  the  ordinary 
demands  upon  the  aerial  photographer.  These  are  single 
objectives  or  pin  points,  mosaic  maps  of  strips  of  territory 
or  large  areas,  oblique  views,  and  stereoscopic  views.  Each 
of  these  presents  its  own  peculiar  problems  influencing 
camera  design. 

Pinpoints  consist  of  such  objects  as  gun  emplacements, 
railway  stations,  ammunition  dumps,  and  other  objects  of 
which  photographs  of  considerable  magnification  are  desired 
for  study.  Here  the  instrumental  requirements  are  suffi- 
cient focal  length  of  lens  to  secure  an  image  of  adequate  size; 
means  for  pointing  the  camera  accurately;  enough  shutter 
speed  to  counterbalance  the  speed  of  the  plane;  sufficiently 
wide  lens  aperture  to  give  adequate  exposure  with  the  shutter 
speed  required;  means  of  supporting  the  camera  to  protect 
it  from  the  vibration  of  the  plane. 

Mosaic  maps  are  built  up  from  a  large  number  of  photo- 
graphs of  adjacent  areas.  In  addition  to  the  above  require- 
ments, mosaic  maps  demand  lenses  free  from  distortion  and 
covering  as  large  a  plate  as  possible,  in  order  to  keep  to  a 
minimum  the  number  of  pictures  needed  to  cover  a  given 
area;  means  for  keeping  the  camera  accurately  vertical,  and 
means  for  changing  the  plates  or  films  and  resetting  the 
shutter  rapidly  enough  to  avoid  gaps  between  successive 


40          AIRPLANE  PHOTOGRAPHY 

pictures.  At  low  altitudes  and  high  ground  speeds  the  interval 
between  exposures  becomes  a  matter  of  only  a  few  seconds. 

Oblique  views  are  made  at  angles  of  from  12  to  35  degrees 
from  the  horizontal,  usually  from  comparatively  low  alti- 
tudes. They  have  been  found  to  be  particularly  suitable  for 
the  use  of  men  who  have  no  training  in  photographic  inter- 
pretation, being  more  like  the  pictures  with  which  the  men 
are  familiar.  Distributed  among  the  infantry  before  an 
attack,  they  have  proved  indispensable  aids  to  the  proper 
knowledge  of  the  ground  to  be  covered.  Tfye  additional  re- 
quirement here  is  for  high  shutter  speed  to  elminate  the  effect 
of  the  relatively  very  rapid  movement  of  the  foreground. 

Stereoscopic  views  are  among  the  most  useful  of  all  air- 
plane pictures.  They  are  made  from  successive  exposures, 
the  separation- of  the  points  of  view  being  obtained  not  by 
two  lenses  at  the  distance  of  the  eyes  apart,  but  by  the 
motion  of  the  plane.  For  this  purpose  the  views  should 
overlap  by  at  least  60  per  cent;  this, therefore, requires  a  very 
short  interval  between  exposures.  For  stereo-oblique  views 
this  may  mean  that  they  are  taken  at  intervals  as  short  as 
one  or  two  seconds. 

Chief  Differences  between  Ground  and  Air  Cameras. — 
Certain  definite  differences  are  thus  seen  to  stand  out  be- 
tween airplane  cameras  and  the  ordinary  kind.  It  is  essential 
that  the  apparatus  for  use  in  the  air  shall  have  high  lens  and 
shutter  speed,  means  for  rapid  changing  of  plates,  and  anti- 
vibration  suspension.  Without  these  features  a  camera  is 
of  little  use  for  aerial  work.  These  requirements  lead  inevi- 
tably to  greater  complexity  of  design,  One  simplification 
over  ground  cameras,  however,  is  brought  about  by  the  fact 
that  all  exposures  are  made  on  objects  beyond  the  practical 
infinity  point  of  the  lens;  consequently,  all  cameras  are  fixed 
focus.  This  fixed  focus  feature  is  a  positive  advantage  in 


THE  CAMERA  41 

construction,  since  it  permits  of  the  simple  rigid  box  form, 
desirable  and  necessary  to  withstand  the  strains  due  to  the 
weight  of  the  lens  and  the  stresses  from  the  plane.  But  with 
the  abandonment  of  all  provision  for  focussing  in  the  air 
must  go  special  care  that  the  material  used  in  constructing 
the  camera  body  is  as  little  subject  as  possible  to  expansion 
and  contraction  with  temperature,  since  there  is  often  a 
drop  of  30  to  40  degrees  Centigrade  from  ground  to  upper 
air.  The  effect  of  change  of  temperature  on  focus  will  be 
treated  in  the  discussion  of  lenses. 

In  addition  to  these  differences,  we  must  keep  in  mind  cer- 
tain requirements  which  are  conditioned  by  the  nature  and 
place  of  aerial  navigation.  Thus  all  mechanical  devices  which 
will  fail  to  function  at  the  low  temperatures  and  pressures  met 
at  high  altitudes  are  entirely  unsuitable.  Experience  has 
shown,  too,  that  we  must  avoid  all  mechanism  depending 
primarily  on  springs  and  on  the  action  of  gravity.  Vibration, 
and  the  motion  of  the  plane  in  all  three  dimensions,  conspire 
to  render  mechanical  motions  unreliable  when  actuated  by 
these  agencies.  All  plate  changing,  shutter  setting,  and 
exposing  operations  should  be  as  nearly  as  possible  positively 
controlled  motions.  Because  of  the  cold  of  the  upper  air 
all  knobs,  levers  and  catches  must  be  made  extra  large  and 
easy  to  handle  with  heavy  gloves.  Circular  knurled  heads 
to  such  parts  as  shutter  setting  movements  are  to  be  avoided 
in  favor  of  bat- wing  keys  or  levers.  Grooves  for  the  recep- 
tion of  magazines  must  be  as  large  and  smooth  as  possible, 
and  guides  to  facilitate  the  magazines'  introduction  should 
be  provided  (Fig.  50).  No  releases  or  adjustments  which 
depend  upon  hearing  or  upon  a  delicate  sense  of  touch  are 
feasible  in  airplane  apparatus.  Wherever  possible,  large 
visible  indicators  of  the  stage  of  the  cycle  of  operations  should 
be  provided.  Loose  parts  are  to  be  shunned,  as  they  are 


42          AIRPLANE  PHOTOGRAPHY 

invariably  lost  in  service.  Complete  operating  instruc- 
tions should  be  placed  on  the  apparatus  wherever  pos- 
sible, to  minimize  the  confusion  due  to  changing  and 
uninstructed  personnel. 

The  Elements  of  the  Airplane  Camera. — Disregarding 
its  means  of  suspension,  the  airplane  camera  proper  consists 
essentially  of  lens,  camera  body,  shutter,  and  plate  or  film 
holding  and  changing  box. 

In  certain  of  the  aerial  cameras  developed  early  in  the 
war  all  of  these  elements  were  built  together  in  a  common 
enclosure.  Later  it  was  generally  recognized  that  a  unit 
system  of  interchangeable  parts  is  preferable.  In  the  case 
of  the  lens  there  arose  various  requirements  for  focal  length, 
from  25  to  120  centimeters,  according  to  the  work  to  be 
done.  Rather  than  use  an  entirely  different  camera  for  each 
different  kind  of  work,  it  is  better  to  have  lenses  of  various 
focal  lengths,  mounted  in  tubes  or  cones,  all  built  to  attach 
to  the  same  camera  body.  In  the  case  of  the  shutter  it  is 
desirable  to  be  able  to  repair  or  calibrate  periodically.  By 
making  the  shutter  a  removable  unit,  the  provision  of  a  few 
spares  does  away  with  the  need  for  putting  the  whole 
camera  out  of  commission.  Similar  considerations  hold 
with  reference  to  other  parts. 

A  further  material  advantage  that  comes  from  making 
airplane  cameras  in  sections  is  the  greater  ease  with  which 
they  are  inserted  in  the  plane,  usually  through  the  openings 
between  diagonal  cross- wires.  It  is  in  fact  only  by  virtue 
of  this  possibility  of  breaking  up  into  small  elements  that 
some  of  the  larger  cameras  could  be  inserted  in  the  common 
types  of  reconnaissance  plane.  Illustrations  of  the  building 
up  of  cameras  from  separate  removable  elements  are  given 
in  the  detailed  discussion  of  the  individual  types. 

Types    of    Airplane    Cameras. — During    the    course   of 


THE   CAMERA  43 

the  war  airplane  cameras  have  been  classified  on  various 
bases,  in  different  services.  In  the  French  service,  where 
the  de  Maria  type  of  camera  was  standardized  early  in  the 
war,  the  usual  classification  was  based  on  focal  length; 
thus  the  standard  cameras  were  spoken  of  as  the  26,  the  50 
and  the  120  (centimeter).  A  further  distinction  was  then 
made  according  to  the  size  of  plate,  this  being  originally 
13X18  centimeters  for  the  26  centimeter,  and  18X24 
centimeters,  for  the  larger  cameras.  In  the  English  service 
the  4X5  inch  plate  was  used  almost  exclusively,  and  their 
various  types  of  cameras  were  known  by  serial  letters — C,  E, 
L,  etc.  Both  these  modes  of  classification  became  inade- 
quate with  the  ultimate  agreement  to  standardize  on  the 
18x24  centimeter  size  for  all  plates,  and  to  carry  lenses  of 
all  focal  lengths  in  interchangeable  elements. 

For  purposes  of  description  and  discussion,  it  is  most 
convenient  to  classify  cameras  according  to  their  method  of 
operation  and  the  sensitive  material  employed.  On  this 
basis  we  may  distinguish  among  cameras  using  plates 
three  kinds — non-automatic  cameras,  semi-automatic  cameras, 
and  automatic  cameras.  We  may  similarly  discuss  film 
cameras,  but  having  treated  the  plate  cameras  comprehen- 
sively, it  will  be  found  that  the  discussion  of  all  types  of  film 
camera  can  be  handled  most  conveniently  by  studying  the 
differences  in  construction  and  operation  introduced  by  the 
characteristics  of  film  as  compared  to  plates. 


CHAPTER  IV 
LENSES  FOR  AERIAL  PHOTOGRAPHY 

General  Considerations. — The  design  and  selection  of 
lenses  for  aerial  photography  present  on  the  whole  no 
problems  not  already  encountered  in  photography  of  the 
more  familiar  sort.  Indeed,  the  lens  problem  in  the  airplane 
camera  is  in  some  particulars  more  simple  than  in  the  ground 
camera.  For  instance,  there  is  no  demand  for  depth  of  focus 
—all  objects  photographed  are  well  beyond  the  usually 
assumed  "infinity  focus"  of  2000  times  the  lens  diameter. 
Such  strictly  scientific  problems  of  design  as  pertain  to 
aerial  photographic  lenses  are  ones  of  degree  rather  than  of 
kind.  Larger  aperture,  greater  covering  power,  smaller 
distortion,  more  exquisite  definition — these  always  will  be 
in  demand,  and  each  progressive  improvement  will  be 
reflected  in  advances  in  the  art  of  aerial  photography.  But 
many  lens  designs  perfected  before  the  war  were  admirably 
suited,  without  any  change  at  all,  for  aerial  cameras. 

Of  the  utmost  seriousness,  however,  with  the  Allies,  was 
the  problem  of  securing  lenses  of  the  desired  types  in  suffi- 
cient numbers.  The  manufacture  of  the  many  varieties  of 
optical  glass  essential  to  modern  photographic  lenses  was 
almost  exclusively  a  German  industry,  which  had  to  be 
learned  and  inaugurated  in  Allied  countries  since  1914.  In 
consequence  of  this  entirely  practical  problem  of  quantity 
production  without  the  glasses  for  which  lens  formulae  were 
at  hand,  some  new  lens  designs  were  produced.  Whether 
any  of  these  possess  merits  which  will  lead  them  to  be  pre- 
ferred over  pre-war  designs,  when  the  latter  can  again  be 
manufactured,  remains  to  be  seen. 
44 


LENSES  45 

While  the  glass  problem  was  still  unsolved,  aerial  cameras 
had  to  be  equipped  with  whatever  lenses  could  be  secured 
by  requisition  from  pre-war  importation  and  manufacture, 
and  later,  with  lenses  designed  to  utilize  those  glasses  whose 
manufacture  had  been  mastered  in  the  allied  countries.  It 
is  important  that  the  historical  aspect  of  this  matter  be  well 
understood  by  the  student  of  aerial  photographic  methods, 
for  the  use  of  these  odd-lot  lenses  reacted  on  the  whole 
design  of  aerial  cameras  and  on  the  methods  of  aerial  photog- 
graphy,  particularly  in  England  and  the  United  States. 
Almost  without  exception  the  available  lenses  were  of  short 
focus,  considered  from  the  aerial  photographic  standpoint; 
that  is,  they  lay  between  eight  and  twelve  inches.  This  set 
a  limit  to  the  size  of  the  airplane  camera,  quite  irrespective  of 
the  demands  made  by  the  nature  of  the  photographic 
problem.  Lenses  of  these  focal  lengths  produced  images 
which,  for  the  usual  heights  of  flying,  were  generally  con- 
sidered too  small,  and  which  were,  therefore,  almost  always 
subsequently  enlarged.  Such  was  the  English  practice, 
which  was  followed  in  the  training  of  aerial  photographers 
in  America,  where  exactly  similar  conditions  held  at  the 
start  with  respect  to  available  lenses.  French  glass  and  lens 
manufacturers  did  succeed  in  supplying  lenses  of  longer 
focus  (50  centimeters),  in  numbers  sufficient  for  their  own 
service,  although  never  with  any  certainty  for  their  allies. 
The  French,  therefore,  almost  from  the  start,  built  their 
cameras  with  lenses  of  long  focus,  and  made  contact  prints 
from  their  negatives. 

Practices  adopted  under  pressure  of  an  emergency  to 
meet  temporary  practical  limitations  often  come  to  dominate 
the  whole  situation.  This  is  particularly  true  of  aerial 
photography  in  the  British  and  American  services.  The 
small  apparatus  built  around  the  stop-gap  short  focus  lenses 


46          AIRPLANE  PHOTOGRAPHY 

fixed  the  plane  designer's  idea  of  an  airplane  camera,  and  the 
space  it  should  occupy.  This  was  directly  reflected  in  the 
designs  of  the  English  planes,  and  the  American  planes 
copied  after  them.  Meanwhile  the  American  photographic 
service  in  France  associated  itself  with  the  French  service, 
adopting  its  methods  and  apparatus,  and  using  French 
planes  whose  designs  were  not  being  followed  in  American 
construction.  The  task  of  harmonizing  the  photographic 
practice  as  taught  in  America,  following  English  lines, 
with  French  practice  as  followed  in  the  theater  of  war,  and  of 
adapting  planes  built  on  English  designs  so  that  they  could 
carry  French  apparatus,  was  a  formidable  one,  not  likely  to 
be  soon  forgotten  by  any  who  had  a  part  in  it. 

Photographic  Lens  Characteristics. — Whole  volumes  have 
been  written  on  the  photographic  lens,  and  on  the  optical 
science  utilized  and  indeed  brought  into  being  by  its  prob- 
lems. Such  works  should  be  consulted  by  those  who  intend 
to  make  a  serious  study  of  the  design  of  lenses  for  aerial 
use.  No  more  can  be  attempted,  no  more  indeed  is  relevant 
here,  than  an  outline  review  of  the  chief  characteristics  and 
errors  of  photographic  lenses,  considering  them  with  special 
reference  to  aerial  needs. 

The  modern  photographic  lens  is,  broadly  speaking,  a 
development  of  the  simple  convex  or  converging  lens.  Its 
function  is  the  same :  to  form  a  real  image  of  objects  placed 
before  it.  But  the  difference  in  performance  between  the 
simple  lens  and  the  modern  photographic  objective  is  enor- 
mous. The  simple  lens  forms  a  clear  image  only  close  to 
its  axis,  for  light  of  a  single  color,  and  as  long  as  its  aperture 
is  kept  quite  small  as  compared  to  the  distance  at  which 
the  image  is  formed.  The  photographic  lens,  on  the  other 
hand,  is  called  upon  to  produce  a  clear  image  with  light  of 
a  wide  range  of  spectral  composition,  sharply  defined  over 


LENSES 


47 


a  flat  surface  of  large  area,  and  it  must  do  this  with  an  aper- 
ture that  is  large  in  comparison  with  the  focal  length, 
whereby  the  amount  of  light  falling  on  the  image  surface 
shall  be  a  maximum.  This  ideal  is  approximated  to  a  really 
extraordinary  degree  by  the  scientific  combination  and 
arrangement  of  lens  elements  made  from  special  kinds  of 
glass  in  the  best  photographic  lenses  of  the  anastigmat  type. 
The  result  is  of  necessity  a  set  of  compromises,  whereby  the 


A, 


FIG.  12. — Diagrammatic  representation  of  spherical  aberration. 

outstanding  errors  are  reduced  to  a  size  judged  permissible 
in  view  of  the  work  the  lens  is  to  do.  These  errors  or  aberra- 
tions are  briefly  reviewed  below,  in  order  that  the  reader 
may  readily  grasp  the  terms  in  which  the  performance  and 
tolerances  in  aerial  lenses  are  described. 

Spherical  Aberration  and  Coma. — Suppose  we  focus  on  a 
screen,  by  means  of  a  simple  convex  lens  the  image  of  a 
distant  point  of  light.  Suppose  for  simplicity  that  this 
image  is  located  on  the  axis  of  the  lens  and  that  light  of  only 
one  color  is  used,  such  as  yellow.  It  will  be  found  that  the 
smallest  image  that  can  be  obtained  is  not  a  point,  but  a 
small  disc.  This  is  due  to  the  fact  that  the  rays  of  light 


48          AIRPLANE  PHOTOGRAPHY 

passing  through  the  outer  portions  of  the  lens  are  bent  more 
than  those  passing  through  the  lens  in  the  region  near  the 
center.  This  effect  is  shown  in  Fig.  12  by  the  usual  mode  of 
representing  it  graphically.  Here  the  figures  1,  2,  3,  4, 
represent  distances  from  the  axis  of  the  lens,  and  the  letters 
Ai,  A2,  A3,  A4,  the  points  of  convergence  of  the  rays  from 
1,  2,  3,  4,  etc.  These  distances  projected  upward  on  to  the 
produced  lens  points  form  a  curve  which  shows  at  a  glance 
the  extent  and  direction  of  the  error  due  to  each  part  of  the 
lens.  This  information  is  of  value  where  the  lens  is  fitted 
with  an  adjustable  diafram.  With  some  types  of  correction 
sharper  definition  may  be  obtained  by  reducing  the  aperture. 
With  others,  however,  diaframing  impairs  definition,  by  de- 
stroying the  balance  between  under  and  over  correction 
which  averages  to  make  a  good  image.  In  aerial  lenses  it 
is  not  customary  to  use  diaframs,  as  all  the  light  possible  is 
desired.  Consequently  the  reduction  of  spherical  aberration 
must  be  accomplished  by  proper  choice  of  lens  elements  and 
their  arrangement. 

Off  the  axis  of  the  lens  the  image  of  a  point  source  takes 
on  an  irregular  shape,  due  to  oblique  spherical  aberration 
or  coma. 

Chromatic  Aberration. — Because  of  the  inherent  proper- 
ties of  the  glass  of  which  it  is  made,  a  simple  collective  lens 
does  not  behave  in  the  same  way  with  respect  to  light  of 
different  colors.  If  one  attempts,  with  such  a  lens,  to  focus 
upon  a  screen  the  image  of  a  distant  white  light,  it  will  be 
found  that  the  blue  rays  will  not  focus  at  the  same  point 
as  the  red  rays,  but  will  come  together  nearer  the  lens. 
Modern  photographic  lenses  are  compounded  of  two  or 
more  kinds  of  glass  in  such  a  way  as  to  largely  eliminate 
this  defect,  the  presence  of  which  is  detrimental  to  good 
definition.  Such  lenses  are  called  achromatic,  and  the 


LENSES  49 

property  of  a  lens  by  virtue  of  which  this  defect  is  eliminated 
is  called  its  chromatic  correction. 

Chromatic  correction  is  never  perfect,  but  two  colors 
of  the  spectrum  can  be  brought  to  a  focus  in  the  same 
plane,  and  to  a  certain  extent  the  departure  of  other 
colors  from  this  plane  can  be  controlled.  Off  the  axis  of 
the  lens  outstanding  chromatic  aberration  results  in  a 
difference  in  the  size  of  images  of  different  colors,  known  as 
lateral  chromatism. 

Like  spherical  aberration,  chromatic  aberration  is  a 
contributing  factor  to  the  size  of  the  image  of  a  point  source, 
which  determines  the  defining  power  of  a  lens.  It  is,  however, 
an  error  whose  effect  is  to  some  extent  dependent  on  the 
kind  of  sensitive  plate  used.  Two  lenses  may  give  images 
of  the  same  size  (in  so  far  as  it  is  governed  by  chromatic 
aberration),  if  a  plate  of  narrow  spectral  sensitiveness  is 
used,  while  giving  images  of  different  size  on  panchromatic 
plates  of  more  extended  color  sensibility.  The  choice  of  the 
region  of  the  spectrum  for  which  chromatic  correction  is  to 
be  made  is  thus  governed  by  the  color  of  the  photographi- 
cally effective  light.  While  in  ordinary  photography  the  blue 
of  the  spectrum  is  most  important,  in  aerial  work  where 
color  filters  are  habitually  used  with  isochromatic  plates 
the  green  is  most  important,  and  color  correction  centered 
about  this  region  constitutes  a  real  difference  of  design 
peculiar  to  aerial  lenses.  Similarly  the  general  use  of  deep 
orange  or  red  filters  with  red  sensitive  plates,  for  heavy  mist 
penetration,  would  call  for  a  shift  of  correction  to  that  part 
of  the  spectrum. 

Astigmatism   and   Covering   Power. — Suppose    the    lens 

forms  at  some  point  off  its  axis  an  image  of  a  cross.    Suppose 

one  of  the  elements  of  the  cross  to  be  on  a  radius  from  the 

center  of  the  field,  the  other  element  parallel  to  a  tangent. 

4 


50          AIRPLANE  PHOTOGRAPHY 

The  rays  forming  the  images  of  these  two  elements  of  the 
cross  are  subject  to  somewhat  different  treatment  in  their 
passage  through  the  lens.  The  curvature  of  the  lens  sur- 
faces is  on  the  whole  greater  with  respect  to  the  rays  from 
the  radial  element  than  to  those  from  the  tangential  element. 
They  are  therefore  refracted  more  strongly  and  come  to  a 
focus  nearer  the  lens.  The  arms  of  the  cross  are  conse- 
quently not  all  in  focus  at  once.  This  error,  termed  astig- 
matism, is  rather  well  shown  in  Fig.  15,  where  the  images  of 
the  outlying  concentric  circles  are  sharp  in  the  radial,  but 
blurred  in  the  tangential  direction. 

Astigmatism  can  be  largely  compensated  for,  and  its 
character  controlled.  The  most  usual  correction  brings 
the  two  images  in  focus  together  both  at  the  axis,  and  on  a 
circle  at  some  distance  out.  This  second  locus  of  coincidence 
may  or  may  not  be  in  the  same  plane  as  the  first,  depending 
on  which  disposition  produces  the  best  average  correction. 
The  mean  between  the  two  foci  determines  the  focal  plane 
of  the  lens,  which  is  in  general  somewhat  curved.  The 
covering  power  of  a  lens  is  given  by  the  size  of  the  field  which 
is  sufficiently  flat  and  free  from  astigmatism  for  the  purpose 
for  which  the  lens  is  used.  This  is  largely  determined  by 
the  astigmatism,  but  the  other  aberrations  are  also  important. 

Illumination. — The  amount  of  light  concentrated  by  the 
lens  on  each  elementary  area  of  the  image  determines  its 
brightness  or  illumination.  The  ideal  image  would,  of  course, 
be  equally  bright  over  its  whole  area  of  good  definition,  and 
for  lenses  of  narrow  angle  this  is  approximately  true.  But 
when  it  is  desired  to  cover  a  wide  angle  the  question  of  illumi- 
nation becomes  serious.  The  relationship  between  angle 
from  the  axis  and  illumination  is  that  illumination  is  pro- 
portional to  the  fourth  power  of  the  cosine  of  the  angle. 
This  relationship  is  shown  in  the  following  table: 


LENSES 


51 


Angle 
0° 
10° 
20° 
30° 
40° 
50° 


Image  brightness 
100     per  cent. 

94.1  per  cent. 

78.0  per  cent. 

56.2  per  cent. 
34.4  per  cent. 

17.1  per  cent. 


If  the  field  of  view  is  60°,  which  corresponds  to  an  18X24 
centimeter  plate  with  a  lens  of  25  centimeter  focus,  the 
brightness  is  only  56  per  cent.,  and  the  necessary  exposure 
at  the  edge  approximately  1.8  times  that  at  the  center. 
This  effect  is  shown  in  Fig.  15.  It  is  very  noticeable  if  the 
exposure  is  so  short  as  to  place  the  outlying  areas  in  the 
under-exposure  period. 


\ 


FIG.  13. — Barrel  and  pin^-cushion  distortion. 

Distortion. — Sometimes  a  lens  is  relatively  free  from  all 
the  aberrations,  mentioned  above,  so  that  it  gives  sharp, 
clear  images  on  the  plate,  yet  these  images  may  not  be 
exactly  similar  to  the  objects  themselves  as  regards  their 
geometrical  proportions;  in  other  words,  the  image  will 
show  distortion.  Lens  distortion  assumes  two  typical 
forms,  illustrated  in  Fig.  13,  which  shows  the  result  of 
photographing  a  square  net-work  with  lenses  suffering  in 
the  one  case  from  "barrel"  distortion  and  in  the  other 
from  "pin-cushion"  distortion.  In  the  first  the  corners  are 
drawn  in  relative  to  the  sides;  in  the  latter  case  the 


AIRPLANE  PHOTOGRAPHY 


sides  are  drawn  in  with  respect  to  the  corners.  Either  sort 
is  a  serious  matter  in  precision  photography,  such  as  aerial 
photographic  mapping  aspires  to  become.  It  must  be 
reduced  to  a  minimum  and  its  amount  must  be  accurately 
known  if  negatives  are  to  be  measured  for  the  precise  location 
of  photographed  objects.  In  general  symmetrical  lenses 
give  less  distortion  than  the  unsymmetrical  (Fig.  14). 

Lens  Testing  and  Tolerances  for  Aerial  Work. — Simple 
and  rapid  comparative  tests  of  lenses  may  be  made  by 


f 

V 

\ 

I 

' 

7  \ 
\  i 

A 
V 

V7Y7 


L\L\ 


a 


FIG.  14. — Arrangement  of  elements  in  two  lenses  suitable  for  aerial  work:  a,  Zeiss  Tessar; 
two  simple  and  one  cemented  components  (unsymmetrical);  b,  Hawk-eye  Aerial;  two  positive 
elements  of  heavy  barium  crown,  two  negative  of  barium  flint,  uncemented  (symmetrical). 

photographing  a  test  chart,  consisting  of  a  large  flat  surface 
on  which  are  drawn  various  combinations  of  geometrical 
figures — lines,  squares,  circles,  etc. — calculated  to  show  up  any 
failures  of  defining  power.  For  testing  aerial  lenses  the 
chart  should  be  as  large  as  possible,  so  that  it  may  be  photo- 
graphed at  a  distance  great  enough  for  the  performance  of 
the  lens  to  be  truly  representative  of  its  behavior  on  an 
object  at  infinite  distance.  This  means  in  practice  a  chart 
of  4  or  5  meters  side,  to  be  photographed  at  a  distance  20 
to  30  times  the  focal  length  of  the  lens. 

A  typical  photograph  of  such  a  chart  is  shown  in  Fig.  15. 
It  reveals  at  a  glance  the  more  conspicuous  lens  errors. 


LENSES 


53 


54          AIRPLANE  PHOTOGRAPHY 

At  the  sides  and  corners  the  concentric  circles  show  the 
lens's  astigmatism,  by  the  clear  definition  of  the  lines  radial 
to  the  center  of  the  field  and  their  blurring  in  the  tangential 
direction.  The  falling  off  in  illumination  with  increasing 
distance  from  the  center  is  also  exhibited;  and  the  blurring 
of  all  detail  outside  the  rectangle  for  which  the  lens  was 
calculated  shows  that  spherical,  chromatic,  and  other 
aberrations  have  become  prohibitively  large. 

But  the  only  complete  test  of  a  lens  is  the  quantitative 
measurement  of  errors  made  on  an  optical  bench.  A  point 
source  of  light,  which  may  at  will  be  made  of  any  color  of 
the  spectrum,  is  used  as  the  object  and  its  image  formed  by 
the  lens  in  a  position  where  it  can  be  accurately  measured 
for  location,  size,  and  shape  by  a  microscope.  A  chart 
giving  the  results  of  such  a  test  is  shown  in  Fig.  16.  In  the 
upper  left-hand  corner  is  shown  the  position  of  the  focus  for 
the  different  colors  of  the  spectrum.  Below  this  is  recorded 
the  lateral  chromatism  at  21  degrees,  in  terms  of  the  differ- 
ence in  focus  for  a  red  and  a  blue  ray.  Below  this  again 
comes  the  distortion,  or  shift  of  the  image  from  its  proper 
position,  for  various  angles  (plotted  at  the  extreme  right) 
from  the  lens  axis.  To  the  right  of  this  is  the  image  size,  at 
each  angle,  and  finally,  to  the  right  of  the  diagram,  are 
plotted  the  distances  of  the  two  astigmatic  foci  from  the 
focal  plane,  together  with  the  mean  of  the  two  foci,  which 
practically  determines  the  shape  of  the  field. 

An  important  point  to  notice  is  that  these  data  are 
uniformly  plotted  in  terms  of  a  lens  of  100  millimeters  focal 
length  irrespective  of  the  actual  focal  length  of  the  lens 
measured.  Thus  this  particular  chart  is  for  a  50  centimeter 
lens  but  would  be  plotted  on  the  same  scale  for  a  25  or  a 
100  centimeter  lens.  Underlying  this  practice  is  the  assump- 
tion that  all  the  characteristics  of  lenses  of  the  same  design 


LENSES 


55 


and  aperture  are  directly  proportional  to  their  focal  length. 
If  this  were  so,  then  a  50  centimeter  lens  would  give  double 
the  size  of  image  that  a  25  centimeter  does,  and  so  on. 
As  a  matter  of  fact,  test  shows  that  the  size  of  the  image 
does  not  increase  so  rapidly  as  the  focal  length;  so  that 
while  the  image  size  for  a  25  centimeter  lens  would  be,  say, 


Blue 


Green 


X40 


Red 


Cofot 


; - 


FIG.  16. — Chart  recording  measurements  of  lens  characteristics. 

.05  millimeters  per  100  millimeters  focal  length,  it  will  be 
only  .03  or  .04  millimeters  per  100  millimeters  focal  length 
for  a  50  centimeter  lens.  The  actual  size  of  a  point  image 
will  therefore  be  greater,  though  not  proportionately  greater. 
The  chart  presents  tests  on  a  good  quality  lens,  and  so 
gives  a  good  idea  of  the  permissible  magnitude  of  the  various 
errors.  In  many  ways  the  most  important  figure  is  that  for 
image  size,  including  as  it  does  the  result  of  all  the  aberra- 


56          AIRPLANE  PHOTOGRAPHY 

tions.  In  the  example  given,  this  varies  from  .075  to  .15 
mm.  actual  size.  For  the  same  type  of  lens  of  25  centi- 
meters focus  this  range  will  be  from  .05  to  .10  mm.  Since 
these  are  commonly  used  focal  lengths,  a  good  average 
figure  for  image  size,  commonly  used  in  aerial  photographic 
calculations,  is  1/10  mm.  In  regard  to  astigmatic  tolerances, 
the  two  astigmatic  foci  should  not  be  separated  at  any  point 
by  more  than  6  to  7  millimeters,  and  the  mean  of  these 
should  not  deviate  from  the  true  flat  field  by  more  than  Y^ 
millimeter,  in  each  case  the  figures  being  based  on  the  con- 
ventional 100  millimeters  focal  length.  Distortion  should 
not  be  over  .08  millimeter  at  18°  or  .20  millimeter  at  24° 
from  the  axis  (per  100  millimeters  focal  length). 

Lens  Aperture. —  In  the  simple  lens  the  aperture  is 
merely  the  diameter.  In  compound  lenses  the  aperture 
is  not  the  linear  opening  but  the  effective  opening  of  an 
internal  diafram.  Photographically,  however,  aperture  has 
come  to  have  a  more  extensive  meaning.  While  in  the 
telescope  the  actual  diameter  of  an  objective  is  perhaps 
the  most  important  figure,  and  in  the  microscope  the  focal 
length,  in  photography  the  really  important  feature  is  the 
amount  of  light  or  illumination.  This  is  determined  by 
lens  opening  and  focal  length  together;  specifically,  by  the 
ratio  of  the  lens  area  to  the  focal  length.  The  common 
system  of  representing  photographic  lens  aperture  is  by  the 
ratio  of  focal  length  to  lens  diameter,  the  lens  being  assumed 
to  be  circular.  Thus  F/5  (often  written  F.5)  indicates  that 
the  diameter  is  one-fifth  the  focal  length. 

Two  points  are  to  be  constantly  borne  in  mind  in  con- 
nection with  this  system  of  representation.  First,  all  lenses 
of  the  same  aperture  (as  so  represented)  give  the  same 
illumination  of  the  plate  (except  for  differences  due  to  loss 
of  light  by  absorption  and  reflection  in  the  lens  system). 


LENSES  57 

This  follows  simply  from  the  fact  that  the  illumination  of 
the  plate  is  directly  proportional  to  the  square  of  the  lens 
diameter,  and  inversely  as  the  square  of  the  focal  length. 
Secondly,  the  illumination  of  the  plate  is  inversely  as  the 
square  of  the  numerical  part  of  the  expression  for  aperture. 
That  is,  lenses  of  aperture  F/4.5  and  F/6  give  images  of 

( 6  Y 

relative  brightness  I  — —  I  =1.78. 
^•4.5' 

What  lens  aperture,  and  therefore  what  image  brightness, 
is  feasible,  is  determined  chiefly  by  the  angular  field  that 
must  be  covered  with  any  given  excellence  of  definition. 
The  largest  aperture  ordinarily  used  for  work  requiring  good 
definition  and  flat  field  free  from  distortion  is  F/4.5.  An- 
astigmatic  lenses  of  this  aperture  cover  an  angle  of  16° 
to  18°  from  the  axis  satisfactorily,  which  corresponds  to  an 
18X24  centimeter  plate  with  a  lens  of  50  centimeters  focus. 
Lenses  with  aperture  as  large  as  F/3.5  were  used  to  some 
extent  in  German  hand  cameras  of  25  centimeters  focal  length, 
with  plates  of  9X12  centimeters.  English  and  American 
lenses  of  this  latter  focal  length  were  commonly  of  aperture 
F/4.5,  designed  to  cover  a  4X5  inch  plate. 

As  a  general  rule  the  greater  the  focal  length  the  smaller 
the  aperture — a  relationship  primarily  due  to  the  difficulty 
of  securing  optical  glass  in  large  pieces.  Thus  while  50 
centimeter  lenses  of  aperture  F/4.5  are  reasonably  easy  to 
manufacture,  the  practicable  aperture  for  quantity  produc- 
tion is  F/6,  and  for  120  centimeter  lenses,  F/10.  This 
means  that  a  very  great  sacrifice  of  illumination  must  be 
faced  to  secure  these  greater  focal  lengths.  As  is  to  be 
expected  from  the  state  of  the  optical  glass  industry,  the 
German  lenses  were  of  generally  larger  aperture  for  the  same 
focal  lengths  than  were  those  of  the  Allies.  Besides  the 
F/3.5  lenses  already  mentioned,  their  50  centimeter  lenses 


58          AIRPLANE  PHOTOGRAPHY 

were  commonly  of  aperture  F/4.8,  their  120  centimeter 
lenses  of  aperture  F/7,  or  of  about  double  the  illuminating 
power  of  the  French  lenses  of  the  same  size. 

Demands  for  large  covering  power  also  result  in  smaller 
aperture.  The  26  centimeter  lenses  used  on  French  hand 
cameras  utilizing  13X18  centimeter  plates  were  commonly 
of  aperture  F/6  or  F/5.6.  The  lens  of  largest  covering  power 
decided  on  for  use  in  the  American  service  was  of  12  inch 
focus,  to  be  used  with  an  18X24  centimeter  plate  (extreme 
angle  26°);  the  largest  satisfactory  aperture  for  this  lens 
is  F/5.6. 

Ordinarily  the  question  of  aperture  is  closely  connected 
with  that  of  diaframs,  whereby  the  lens  aperture  may  be 
reduced  at  will.  Diaframs  have  been  very  little  used  in 
aerial  photography.  All  the  aperture  that  can  be  obtained 
and  more  is  needed  to  secure  adequate  photographic  action 
with  the  short  exposures  required  under  the  conditions  of 
rapid  motion  and  vibration  peculiar  to  the  airplane.  Any 
excess  of  light,  over  the  minimum  necessary  to  secure  proper 
photographic  action,  is  far  better  offset  by  increase  of  shutter 
speed  or  by  introduction  of  a  color  filter.  For  this  reason 
American  aerial  lenses  were  made  without  diaframs.  In 
the  German  cameras,  however,  adjustable  diaframs  are 
provided  (Fig.  43),  controlled  from  the  top  of  the  camera 
by  a  rack  and  pinion.  In  the  camera  most  used  in  the  Italian 
service  an  adjustable  diafram  is  provided,  but  this  is  occa- 
sioned by  the  employment  of  a  between-the-lens  shutter  of 
fixed  speed,  so  that  the  only  way  exposure  can  be  regulated 
is  by  aperture  variation,  a  method  which  has  little  to 
.recommend  it. 

The  Question  of  Focal  Length. — In  aerial  photography 
the  lens  is  invariably  used  at  fixed,  infinity,  focus.  Under 
these  conditions  the  simple  relationship  holds  that  the  size 


LENSES  59 

of  the  image  is  directly  proportional  to  the  focal  length  and 
inversely  proportional  to  the  altitude.  If  any  chosen  scale 
is  desired  "for  the  picture  the  choice  of  focal  length  is  deter- 
mined by  the  height  at  which  it  is  necessary  to  fly.  This 
at  least  would  be  the  case  were  there  no  limitation  to  the 
practicable  focal  length — which  means  camera  size — and 
were  one  limited  to  the  original  size  of  the  picture  as  taken; 
that  is,  were  the  process  of  enlargement  not  available.  But 
the  possibility  of  using  the  enlarging  process  brings  in  other 
questions:  Is  the  defining  power  of  a  short  focus  lens  as 
good  in  proportion  to  its  focal  length  as  that  of  a  long  focus 
lens?  If  so  a  perfect  enlargement  from  a  negative  made  by 
a  short  focus  lens  would  be  identical  with  a  contact  print 
from  a  negative  made  with  a  lens  of  longer  focus.  Is  defining 
power  lost  in  the  enlarging  process  with  its  necessary  em- 
ployment of  a  lens  which  has  its  own  errors  of  definition 
and  which  must  be  accurately  focussed? 

Certain  factors  which  enter  into  comparisons  of  this 
sort  in  other  lines  of  work,  such  as  astronomical  photography, 
play  little  part  here.  These  are,  first,  the  optical  resolving 
power  of  the  lens,  which  is  conditioned  by  the  phenomena  of 
diffraction,  and  is  directly  as  the  diameter;  and,  second, 
the  size  of  the  grain  of  the  plate  emulsion.  The  first  of  these 
does  not  enter  directly,  because  the  size  of  a  point  image 
on  the  axis  of  the  lens,  due  merely  to  diffraction,  is  very 
much  less  than  that  given  by  any  photographic  lens  which 
has  been  calculated  to  give  definition  over  a  large  field,  instead 
of  the  minute  field  of  the  telescope.  Yet  it  may  contribute 
toward  somewhat  better  definition  with  a  long  focus  lens 
because  of  the  actually  larger  diameter  of  such  lenses.  The 
second  factor  is  not  important,  because,  as  will  be  seen  later, 
the  resolving  power  of  the  plates  suitable  for  aerial  photog- 
raphy is  considerably  greater  than  that  of  the  lens.  The 


60          AIRPLANE  PHOTOGRAPHY 

emulsion  grain  is  in  fact  only  a  quarter  or  a  fifth  the  size  of 
the  image  as  given  by  a  25  centimeter  lens,  and  enlargements 
of  more  than  two  or  three  times  are  rarely  wanted. 

A  series  of  experiments  was  made  for  the  U.  S.  Air  Service 
to  test  out  these  questions,  using  a  number  of  representa- 
tive lenses  of  all  focal  lengths,  both  at  their  working  aper- 
tures and  at  identical  apertures  for  all.  With  regard  to  lens 
defining  power,  as  shown  by  the  size  of  a  point -image,  the 
answer  has  already  been  reported  in  a  previous  section. 
Lenses  of  long  focus  give  a  relatively  smaller  image  than 
lenses  of  the  same  design  of  short  focus.  In  regard  to  the 
whole  process  of  making  a  small  negative  and  enlarging  it, 
the  loss  of  definition  is  quite  marked,  as  compared  to  the 
pictures  of  the  same  scale  made  by  contact  printing  from 
negatives  taken  with  longer  focus  lenses. 

This  answer  is  clear-cut  only  for  lenses  calculated  to 
give  the  same  angular  field.  Thus  a  10  inch  lens  covering  a 
4X5  inch  plate  has  about  the  same  angle  as  a  50  centimeter 
lens  for  an  18X24  centimeter  plate.  When,  however,  it 
comes  to  the  longer  foci,  such  as  120  centimeters,  the  practi- 
cal limitation  to  plate  size  (18X24  cm.)  has  been  passed, 
and  the  angular  field  is  less  than  half  that  of  the  50  centi- 
meter lens.  The  120  centimeter  lens  need  only  be  designed 
for  this  small  angle,  with  consequent  greater  opportunities 
for  reduction  of  spherical  aberration.  It  is  therefore  an 
open  question  whether  a  50  centimeter  lens  designed  to 

50 
cover  a  plate  of  linear  dimensions  — •  times  that  used  with 

the  regular  50  centimeter  lens  could  not  be  produced  of  such 
quality  that  it  would  yield  enlargements  equal  to  contacts 
from  a  120  centimeter  lens.  If  so,  lenses  of  larger  aperture 
could  be  used,  and  a  considerable  saving  in  space  require- 
ments effected. 


LENSES  61 

Focal  lengths  during  the  Great  War  were  decided  by  the 
nature  of  the  military  detail  which  was  to  be  revealed  and 
by  the  altitudes  to  which  flying  was  restricted  in  military 
operations.  In  the  first  three  years  of  the  war  the  develop- 
ment of  defences  against  aircraft  forced  planes  to  mount 
steadily  higher,  so  that  the  original  three  or  four  thousand 
feet  were  pushed,  to  15,000, 18,000,  and  even  higher.  Lenses 
of  long  focus  were  in  demand,  leading  ultimately  to  the  use 
of  some  of  as  much  as  120  centimeters  (Fig.  41).  In  the  last 
months  of  the  war  the  resumption  of  open  fighting  made 
minute  recording  of  trench  details  of  less  weight,  while  the 
preponderance  of  allied  air  strength  permitted  lower  flying. 
In  consequence,  lenses  of  shorter  focus  and  wider  angle  came 
to  the  fore,  suitable  for  quick  reconnaissance  of  the  main 
features  of  new  country.  At  the  close  of  the  war  the  follow- 
ing focal  lengths  were  standard  in  the  U.  S.  Air  Service,  and 
may  be  considered  as  well-suited  for  military  needs.  Peace 
may  develop  quite  different  requirements. 

Focal  length  Aperture  Plate  size 

10  inch  F/4.5  4X5  inch 

26cm.  F/6  13X18  cm. 

12  inch  F/5.6  18X24  cm. 

20  inch  F/6.3  to  F/4.5  18X24  cm. 

48  inch  F/10  to  F/8  18X24  cm, 

The  question  of  the  use  of  telephoto  lenses  in  place  cf 
lenses  of  long  focus  is  frequently  raised.  Lenses  of  this  type 
combine  a  diverging  (concave)  element  with  the  normal 
converging  system,  whereby  the  effect  of  a  long  focus  is 
secured  without  an  equivalent  lens-to-plate  distance.  This 
reduction  in  "back  focus"  may  be  from  a  quarter  to  a  half. 
Were  it  possible  to  obtain  the  same  definition  with  telephoto 
lenses  as  with  lenses  of  the  same  equivalent  focus,  they 
would  indeed  be  eminently  suitable  for  aerial  work  because 


62          AIRPLANE  PHOTOGRAPHY 

of  their  economy  of  length.  But  experience  thus  far  has 
shown  that  the  performance  of  telephoto  lenses,  as  to  defini- 
tion and  freedom  from  distortion,  is  distinctly  inferior,  so  that 
it  is  best  to  hold  to  the  long  focus  lens  of  the  ordinary  type. 

Lenses  Suitable  for  Aerial  Photography. — Among  the 
very  large  number  of  modern  anastigmat  lenses  many  were 
found  suitable  for  airplane  cameras  and  were  used  exten- 
sively in  the  war.  A  partial  list  follows :  The  Cooke  Aviar, 
The  Carl  Zeiss  Tessar,  the  Goerz  Dogmar,  the  Hawkeye 
Aerial,  the  Bausch  and  Lomb  Series  Ic  and  lib  Tessars,  the 
Aldis  Triplet,  the  Berthiot  Olor. 

The  Question  of  Plate  Size  and  Shape. — Plate  size  is 
determined  by  a  number  of  considerations,  scientific  and 
practical.  If  the  type  of  lens  is  fixed  by  requirements  as  to 
definition,  then  the  dimensions  of  the  plate  are  limited  by 
the  covering  power.  From  the  standpoint  of  economy  of 
flights  and  of  ease  of  recognizing  the  locality  represented  in  a 
negative,  by  its  inclusion  of  known  points,  lenses  of  as  wide 
angle  as  possible  should  be  used.  If  the  focus  is  long,  this 
means  large  plates,  which  are  bulky  and  heavy.  If  the 
finest  rendering  of  detail  is  not  required  a  smaller  scale 
may  be  employed,  utilizing  short  focus  lenses  and  correspond- 
ingly smaller  plates.  Thus  a  six  inch  focus  lens  on  a  4  X  5  inch 
plate  would  be  as  good  from  the  standpoint  of  angular  field 
as  a  12  inch  on  an  8X10  inch  plate.  This  is  apt  to  be  the 
condition  with  respect  to  most  peace-time  aerial  photography, 
which  may  be  expected  to  free  itself  quickly  from  the  huge 
plates  and  cameras  of  war  origin. 

For  work  in  which  great  freedom  from  distortion  of  any 
sort  is  imperative,  small  plates  will  be  necessary,  for  two 
reasons.  One  is  that  the  characteristic  lens  distortions  are 
largely  confined  to  the  outlying  portions  of  the  field.  The 
other  is  that  a  wide  angle  of  view  inevitably  means  that  all 


LENSES  63 

objects  of  any  elevation  at  the  edge  of  the  picture  are  shown 
partly  in  face  as  well  as  in  plan,  which  prevents  satisfactory 
joining  of  successive  views  (Fig.  128) .  In  making  a  mosaic 
map  of  a  city,  if  a  wide  angle  lens  is  employed  with  large 
plates,  the  buildings  lying  along  the  junctions  of  the  prints 
can  be  matched  up  only  for  one  level.  If  this  is  the  ground 
level,  as  it  would  be  to  keep  the  scale  of  the  map  correct, 
the  roofs  will  have  to  be  sacrificed.  In  extreme  cases  a 
house  at  the  edge  of  a  junction  may  even  show  merely  as  a 
front  and  rear,  with  no  roof,  while  in  any  case  the  abrupt 
change  at  these  edges  from  seeing  one  side  of  all  objects  to 
seeing  the  opposite  side  is  not  pleasing. 

The  table  in  a  preceding  section  gives  the  relation  of 
plate  size  to  focal  length  found  best  on  the  whole  for  military 
needs.  Deviations  from  these  proportions  in  both  directions 
are  met  with.  In  the  English  service  the  LB  camera,  which 
uses  4X5  inch  plates,  is  equipped  with  lenses  of  various  focal 
lengths,  up  to  20  inches.  The  German  practice,  as  well  as 
the  Italian,  was  almost  uniform  use  of  13X18  centimeter 
plates  for  all  focal  lengths.  Toward  the  end  of  the  war, 
however,  some  German  cameras  of  50  centimeter  focal 
length  were  in  use  employing  plates  24X30  centimeters. 

It  will  be  recognized  that  these  plate  sizes  are  chosen 
from  those  in  common  use  before  the  war.  A  similar  obser- 
vation holds  with  even  greater  force  on  the  question  of 
plate  shape.  Current  plate  shapes  have  been  chosen  chiefly 
with  reference  to  securing  pleasing  or  artistic  effects  with  the 
common  types  of  pictures  taken  on  the  ground.  These 
shapes  are  not  necessarily  the  best  for  aerial  photography. 
Indeed  the  whole  question  of  plate  shape  should  be  taken  up 
from  the  beginning,  with  direct  reference  to  the  problems  of 
aerial  photography  and  photographic  apparatus. 

A  few  illustrations  will  make  this  clear,  taking  Fig.  17 


64 


AIRPLANE  PHOTOGRAPHY 


as  a  basis.  If  it  is  desired  to  do  spotting  (the  photography 
of  single  objectives),  the  best  plate  shape  would  be  circular, 
for  that  shape  utilizes  the  entire  covering  area  of  the  lens. 
If  it  is  desired  to  make  successive  overlapping  pictures, 
either  for  mapping,  or  for  the  production  of  stereoscopic 
pairs,  a  rectangular  shape  is  indicated.  If  the  process  of 
plate  changing  is  difficult  or  slow,  it  is  advisable,  in  order 

to  give  maximum  time  for 
this  operation,  to  have  the 
long  side  of  the  rectangle 
parallel  to  the  line  of  flight 
(indicated  by  the  arrow).  If 
economy  of  flights  is  a  con- 
sideration, as  in  making  a  mo- 
saic map  of  a  large  area,  it  is 
advantageous  to  have  as  wide 
a  plate  as  the  covering  power 
of  the  lens  will  permit.  Ref- 
erence to  Fig.  17  shows  that 
this  means  a  plate  of  small  di- 
mensions in  the  direction  of 
flight.  If  the  changing  of 
plates  or  film  is  quick  and 
easy,  the  maximum  use  of  the  lens's  covering  power  is  made 
by  such  a  rectangle  whose  long  side  approximates  the  dimen- 
sions of  the  lens  field  diameter.  This  is  in  fact  the  choice 
made  in  the  German  film  mapping  camera  (Figs.  61  and  63), 
whose  picture  is  6  X 24  centimeters.  An  objection  to  this  from 
the  pictorial  side,  lies  in  the  many  junction  lines  cutting  up 
the  mosaic.  Another  objection,  if  the  plane  does  not  hold 
a  steady  course,  is  the  failure  to  make  overlaps  on  a  turn. 
(Fig.  62.)  Here  as  everywhere  the  problem  is  to  decide  on 
the  most  practical  compromise  between  all  requirements. 


FIG.  17. — Possible  choices  of  plate  shape. 


LENSES  65 

Focussing. — The  process  of  focussing  aerial  cameras 
was  at  first  deemed  a  mystery,  though  undeservedly  so. 
A  belief  was  long  current  that  "ground"  focus  and  "air" 
focus  differ.  In  other  words,  that  a  camera  focussed  upon  a 
distant  object  on  the  ground  would  not  be  in  focus  for  an 
object  the  same  distance  below  the  camera  when  in  the 
plane.  Belief  in  this  mysterious  difference  went  so  far  that 
certain  instruction  books  describe  in  detail  the  process  of 
focussing  a  camera  by  trial  exposures  from  the  air. 

Careful  laboratory  tests  performed  for  the  U.  S.  Air 
Service  showed  that  neither  low  temperature  nor  low  pres- 
sure, such  as  would  be  met  at  high  altitudes,  alter  the  focus 
of  any  ordinary  lens  by  a  significant  amount,  and  that  the 
possible  contraction  of  the  camera  body  was  of  negligible 
effect  on  the  focus  (not  more  than  2iro  Per  cent,  per  degree 
centigrade  with  a  metal  camera).  In  complete  harmony 
with  these  tests  has  been  the  experience  that  if  the  ground 
focussing  is  done  carefully,  by  accurate  means,  then  the  air 
focus  is  correct.  The  whole  matter  thus  becomes  one  of 
precision  focussing. 

The  best  method,  applicable  if  the  air  is  steady,  is  to 
focus  by  parallax.  The  ground  glass  focussing  screen  is 
marked  in  the  center  with  a  pencilled  cross.  Over  this  is 
mounted,  with  Canada  balsam,  a  thin  microscope  cover- 
glass.  The  camera  is  directed  on  an  object  a  mile  or  more 
away,  and  the  image  formed  by  the  lens  is  examined  by  a 
magnifying  glass  through  the  virtual  hole  formed  by  the 
affixed  cover-glass.  With  the  pencil  line  in  focus  the  head 
is  moved  from  side  to  side.  If  the  image  and  pencil  mark 
coincide  they  will  move  together  as  the  head  is  moved.  If 
the  image  moves  away  from  the  pencil  mark  and  in  the 
same  direction  as  the  eye  moves,  the  image  is  too  near  the 
lens.  If  the  image  moves  away  in  the  opposite  direction  to 
5 


66         AIRPLANE  PHOTOGRAPHY 

the  motion  of  the  eye,  it  is  too  far  from  the  lens.  In  either 
case  the  focus  is  to  be  corrected  accordingly. 

In  place  of  a  distant  object,  which  may  waver  with  the 
motion  of  the  air,  we  may  use  an  image  placed  at  infinity 
by  optical  means.  The  collimator,  an  instrument  for  doing 
this,  consists  of  a  test  object  (lines,  circles,  etc.)  placed 
accurately  at  the  focus  of  a  telescope  objective.  The 
camera  lens  is  placed  against  this  and  focussed  by  parallax, 
as  with  a  distant  object.  Collimators  are  employed  in  camera 
factories,  and  should  be  part  of  the  equipment  of  base  labora- 
tories where  repairing  and  overhauling  of  cameras  is  done. 

Lens  Mounts. — All  that  is  required  for  the  mounting  of 
an  aerial  camera  lens  is  a  rigid  platform,  with  provision  for 
enough  motion  of  the  lens  to  adjust  its  focus  accurately. 
As  already  explained,  the  lens  works  at  fixed,  infinity, 
focus,  and  therefore  needs  no  adjustment  during  use.  It 
must  be  held  far  more  rigidly  than  would  be  possible  by  the 
bellows,  which  is  an  almost  invariable  adjunct  of  focussing 
cameras.  The  use  of  ordinary  types  of  hand  cameras  on  a 
plane  is  rarely  successful  just  because  of  the  bellows,  which 
is  strained  and  rattled  by  the  rush  of  wind. 

The  lens  mountings  thus  far  used  have  been  simple 
affairs.  In  the  French  cameras  the  lens  is  merely  screwed 
into  a  flange  which  in  turn  is  fastened  by  screws  to  a  plat- 
form in  the  camera  body.  Adjustment  for  focussing  is  not 
provided;  instead,  the  flange  is  raised  on  thin  metal  rings  or 
washers,  cut  of  such  thickness  by  trial  as  to  bring  the  lens 
to  focus,  once  and  for  all. 

The  IT.  S.  Air  Service  method  of  mounting  is  to  provide 
the  lens  barrel  with  a  long  thread,  which  screws  into  a 
flange  that  in  turn  is  mounted  on  a  platform  in  the  camera 
cone,  by  means  of  thumb-screws.  The  lens  is  focussed  by 
screwing  in  and  out,  and  then  clamped  by  a  screw  through 


LENSES  67 

the  side,  bearing  on  the  thread.  The  whole  mount  may  be 
quickly  removed  by  loosening  the  thumb-screws,  and  once 
focussed  in  one  cone,  can  be  transferred  to  another  similar, 
machine-made  cone  without  change  of  focus.  Fig.  18 
shows  a  20  inch  lens  mounted  in  this  manner.  The  photo- 
graph shows  as  well  the  ring  on  the  front  of  the  lens  by 
means  of  which  circular  color  filters  may  be  held  in  place. 


FIG.  18. — 50  centimeter  F/6  lens  in  U.  S.  standard  mount,  showing  color  filter  retaining  ring  and  catch. 

This  ring  screws  down  on  the  filter,  and  the  catch  is  dropped 
into  the  nearest  vertical  groove  to  the  tight  position. 

A  somewhat  different  and  better  method  of  tightening 
the  lens  in  the  flange,  when  focussed,  has  been  adopted  in  the 
English  lens  mount,  which  is  in  general  similar  to  the 
American.  The  threaded  part  of  the  flange  is  split  by  a  slot 
cut  parallel  to  the  flange  base,  and  a  screw  is  run  into  the  flange 
from  the  front,  through  the  split  portion.  By  tightening 
this  screw,  which  is  always  accessible,  the  split  part  of  the 
flange  is  squeezed  together,  thus  rigidly  holding  the  lens  barrel. 


CHAPTER  V 
THE  SHUTTER 

Permissible  Exposure  in  Airplane  Photography. — A 
definite  limitation  to  the  length  of  exposure  in  airplane 
cameras  is  set  by  the  motion  of  the  plane.  If  we  represent 
the  speed  of  the  plane  by  S,  the  altitude  of  the  plane  by  A, 
and  the  focal  length  of  the  lens  by  F,  we  obtain  at  once 
from  the  diagram  (Fig.  19),  that  s,  the  rate  of  movement  of 
the  image  on  the  plate,  is  given  by  the  relation, 

iS  =A 

If  we  call  the  permissible  movement  d,  then  the  permis- 
sible exposure  time,  t,  is  given  by  the  relation — 

=  7  =  FS 

As  a  representative  numerical  case,  expressing  all 
quantities  in  centimeters  and  in  centimeters  per  second, 

20,000,000, 
let  F  =  50,  S**  — — — —  (200  kilometers  per  hour),  and 

A  =  300,000,  then 

50  X  20,000,000 
^=  300,000  X  3600  ='9centimeterS 

If  we  take  for  the  permissible  undetectable  movement, 
.01  centimeter,  which  is,  as  has  been  shown,  a  reasonable 
figure  for  lens  defining  power,  we  have,  then,  that  the  longest 
permissible  exposure  is  .011  second — in  round  numbers, 
one-hundredth . 

In  flying  with  a  slow  plane,  or  in  flying  against  the  wind, 
the  exposure  can  sometimes  be  increased  to  as  much  as 
double  this  length.  Diminishing  F  would  similarly  extend 
the  allowable  exposure,  but  the  ratio  of  F  to  A  approximates 

68 


THE  SHUTTER 


69 


to  a  constant  in  actual  practice;  in  other  words,  a  certain 
resolution  and  size  of  image  have  been  found  desirable. 
If  flying  is  forced  higher,  a  longer  focus  lens  is  used ;  if  lower 
flying  is  possible,  a  lens  of  shorter  focus.  This  relationship 


FIG.  19. — Relative  motion  of  plane  and  photographic  image. 

has,  of  course,  been  derived  from  war-time  experience. 
Probably  much  of  the  prospective  peace-time  mapping  work 
will  impose  substantially  easier  requirements  as  to  definition 
and  will  thus  allow  longer  exposures. 

For  low  oblique  views  the  longest  exposure  is  much  less. 
Taking  45  degrees  as  a  representative  angle  for  the  fore- 
ground, and  500  meters  as  a  representative  height,  the  value 
of  t  becomes 


70          AIRPLANE  PHOTOGRAPHY 

These  figures  will  illustrate  two  important  points: 
they  show  how  severe  is  the  limitation  as  to  exposure,  with 
the  consequent  heavy  demand  on  lens  and  sensitive  material 
speed;  and  they  show  how  important  it  is  to  secure  a 
shutter  with  the  maximum  light-giving  power  for  a  specified 
length  of  exposure.  This  leads  to  a  study  of  the  character- 
istics as  to  efficiency  of  the  two  common  types  of  shutter, 
namely,  shutters  at  or  between  the  lens,  and  focal-plane  shutters. 

Characteristics  of  Shutters  Located  at  the  Lens. — Of  the 
various  shutters  located  at  the  lens  the  most  common  is  the 
type  that  is  clumsily  but  descriptively  termed  the  "between- 
the-lens"  shutter.  This  is  composed  of  thin  hard  rubber  or 
metal  leaves  or  sectors  which  overlap  and  which  are  pulled 
open  to  make  the  exposure.  It  may  require  two  operations, 
one  for  setting  and  one  for  exposing,  or  it  may,  as  in  some 
makes,  set  and  expose  by  a  single  motion.  Clock  escape- 
ments, or  some  form  of  frictional  resistance,  are  depended  on 
to  control  the  interval  between  opening  and  closing.  This 
shutter  is  the  one  almost  universally  employed  on  small 
hand  cameras  and  on  all  lenses  up  to  about  two  inches 
diameter.  It  gives  speeds  sometimes  marked  as  high  as 
3-^0  second,  although  usually  not  over  y^-Q  on  actual  test. 

Between-the-lens  shutters  have  been  used  to  some  extent 
on  the  shorter  focus  (up  to  25  centimeter)  aerial  cameras, 
notably  in  the  Italian  service.  They  suffer,  however,  from 
two  limitations.  In  the  first  place  we  have  not  yet  solved  the 
mechanical  problems  met  with  in  trying  to  make  the  shutter 
of  large  size  (as  for  50  centimeter  F/6  lenses)  at  the  same 
time  to  give  high  speeds.  In  the  second  place  the  efficiency 
of  the  type  is  low  because  a  large  part  of  the  exposure  time  is 
occupied  by  the  opening  and  closing  of  the  sectors. 

If  we  define  the  efficiency  of  a  shutter  as  the  ratio  of  the 
amount  of  light  it  transmits  during  the  exposure  to  the 


THE  SHUTTER 


71 


amount  of  light  it  would  transmit  were  it  wide  open  during 
the  whole  period,  then  the  efficiency  of  the  ordinary  bet ween- 
the-lens  shutter  is  of  the  order  of  60  per  cent.  This  means 
1.6  times  the  motion  of  the  image  for  the  same  photographic 
action  that  we  should  have  with  a  perfect  shutter.  The 
accompanying  photographic  record  (Fig.  20)  of  the  opening 
and  closing  process  of  this  type  of  shutter  clearly  illustrates 
its  deficiencies. 

Characteristics  of  the  Focal=Plane  Shutter. — Long  before 
the  days  of  aerial  photography  the  problem  of  a  high- 


HG.  20. — Effective  lens  opening  at  equal  intervals  of  time:    (a)  during  focal  plane  shutter  exposure; 
(6)  during  between-the-lens  shutter  exposure. 

efficiency  high-speed  shutter  for  photographing  moving 
objects  on  the  ground — railway  trains  or  racing  automo- 
biles— had  already  led  to  the  development  of  the  focal-plane 
shutter.  This  is  a  type  peculiarly  adapted  to  the  problems 
of  the  airplane  camera.  It  consists  essentially  of  a  curtain, 
running  at  high  speed  close  to  the  photographic  plate,  the 
exposure  being  given  by  a  narrow  rectangular  slot. 

If  the  focal-plane  shutter  is  in  virtual  contact  with  the 
sensitive  surface  the  efficiency,  as  defined  above,  is  100  per 
cent.,  since  the  whole  cone  of  rays  from  the  lens  illuminates 
the  plate  during  the  whole  time  of  exposure.  But  if  the 
curtain  is  not  carried  close  to  the  plate  the  efficiency  falls 


72          AIRPLANE  PHOTOGRAPHY 

off  rapidly  with  distance,  especially  so  for  small  apertures 
of  the  slot. 

The  efficiency  of  the  focal-plane  shutter  may  be  calculated 
as  follows :    Let  the  focal  length  of  the  lens  be  F,  its  diameter 


FIG.  21. — Calculation  of  focal  plane  shutter  efficiency. 

be  F/N,  the  width  of  the  slot  be  a,  and  the  distance  from 
plate  to  curtain  d  (Fig.  21).  Now  if  the  curtain  is  moving 
at  a  uniform  speed,  the  time  taken  for  the  slot  to  traverse  the 
whole  cone  of  rays,  from  the  instant  it  enters  till  the  instant 
it  leaves,  will  be  directly  proportional  to 


THE  SHUTTER 


73 


F\N 

If  the  curtain  were  in  contact  with  the  plate  the  time 
taken  for  the  same  amount  of  light  to  reach  the  sensitive 
surface  would  be  proportional  to  a.  Again  defining  shutter 
efficiency  as  the  ratio  of  the  light  transmitted  to  what  would 
have  been  transmitted  were  the  shutter  fully  open  for  the 
total  time  of  exposure,  the  efficiency,  E,  is  given  at  once  by 
the  expression— 


As  an  example  let  the  lens  aperture  be  F/6,  so  that  N=  6; 
let  d=l9  and  a=l,  then  E=%.     In  the  French  de  Maria 


Percentage  of  total     5* 
light  from  /ens  reaching   so 
o/ofv  at  any  time  during 
exposure 


U.NS  APERTURE 
f/S 

SLIT  APERTURE- 


d=  distance,  curtain  to  plafo. 


6-  total  duration  of  exposure  — * 

FIG.  22. — Characteristics  of  focal  plane  shutter. 

cameras,  where  d=4  centimeters,  E=QO  per  cent,  for  the 
aperture  assumed,  which  is  representative.  Fig.  22  exhibits 
diagrammatically  the  chief  characteristics  of  the  focal 
plane  shutter. 

In  view  of  the  necessity  for  some  distance  between  shutter 


74          AIRPLANE  PHOTOGRAPHY 

and  plate  it  is  obviously  important  to  keep  a  as  large  as 
possible,  depending  for  the  requisite  shutter  speed  on  the 
velocity  of  the  curtain.  Large  aperture  and  high  curtain 
speed  are  also  found  to  be  desirable  when  we  consider  the 
distortion  produced  by  the  focal-plane  shutter. 

Distortions  Produced  by  the  Focal=plane  Shutter.— 
While  the  time  of  exposure  of  any  point  on  the  plate  can, 
with  the  focal -plane  shutter,  easily  be  made  YiTo  second  or 
less,  the  whole  period  during  which  the  shutter  is  moving  is 
much  greater  than  this.  For  instance,  a  1  centimeter  opening 
which  gives  y^-  second  exposure  takes  yV  second  to  move 
across  a  10  centimeter  plate,  or  nearly  -5-  second  for  an  18 
centimeter  plate.  With  a  moving  airplane  this  means  that 
the  point  of  view  at  the  end  of  the  exposure  has  moved 
forward  compared  to  that  at  the  beginning,  by  the  amount 
of  motion  of  the  plane  in  the  interval.  If  the  shutter  moves 
in  the  direction  of  motion  of  the  plane  the  image  will  be 
magnified;  if  in  the  opposite  direction,  it  will  be  compressed 
along  the  axis  of  motion.  The  amount  of  this  distortion  is 
calculated  as  follows: 

Let  the  velocity  of  the  plane  be  V,  and  that  of  the  shutter 
be  v.  Let  the  focal  length  of  the  camera  be  F,  and  the 
altitude  A.  If  the  camera  were  stationary,  a  plate  of  length 
/  would  receive  on  its  surface  an  image  corresponding  to  a 
distance  jXl  on  the  ground.  Due  to  the  motion  of  the 

shutter  the  end  of  the  exposure  occurs  at  a  time  —  after  the 
start.  In  this  time  the  plane  has  moved  a  distance  V  X~> 

hence  the  point  photographed  at  the  end  of  the  shutter 

VI 
travel  is  —  within  or  beyond  the  original  space  covered 

by  the  plate,  depending  on  the  direction  of  motion  of  the 
curtain.  The  distortion,  Z),  is  given  by  the  ratio  of  this  dis- 


THE  SHUTTER  75 

tance  to  the  length  corresponding  to  the  normal  stationary 
field  of  view: 


When  F  =  200  kilometers  per  hour,  #  =  100  centimeters 
per  second,  F  =  50  centimeters,  ^4  =  3000  meters,  we  have  — 

20,000,000  X  50  1 

D  =  3600X100X300,000  =  aPProxmately  Too 

Or  if  the  actual  distance  error  on  the  ground  is  desired, 

—  =  10.8  meters 

v 

As  a  percentage  error  this  one  per  cent,  is  small  compared 
with  other  uncertainties,  such  as  film  shrinkage  or  the  error 
of  level  of  the  camera.  As  an  absolute  error  in  surveying, 
thirty  feet  is,  of  course,  excessive. 

The  distortion  is  diminished  for  any  specified  shutter 
speed  by  making  the  speed  of  travel  of  the  curtain  as  large 
as  possible  and  by  correspondingly  increasing  the  aperture. 
In  connection  with  film  cameras,  another  solution  which 
has  been  suggested  is  to  move  the  film  continuously  during 
the  exposure  in  the  direction  of  the  plane's  motion.  The 
requisite  speed  of  the  film  vr  to  eliminate  distortion  is  given 

by  the  relation  : 

/;   £ 
V      A 

For  the  values  of  F,  F,  and  A  used  above,  v'  =  .92  centi- 
meters per  second.  This  speed  is  clearly  that  which  holds 
the  image  stationary  on  the  film  —  a  fact  which  suggests 
another  object  for  such  movement,  namely,  to  permit  of 
longer  exposures. 


76          AIRPLANE  PHOTOGRAPHY 

The  effect  of  focal  plane  distortion  may  be  averaged  out 
in  the  making  of  strip  maps,  if  the  shutter  is  constructed 
so  as  to  move  in  opposite  directions  on  successive  exposures. 
The  first  picture  will  be  magnified,  the  second  compressed, 
and  so  on,  but  a  strip  formed  of  accurately  juxtaposed 
pictures  will  be  substantially  accurate  in  over-all  length. 
Such  a  shutter  is  embodied  in  one  of  the  German  film 
cameras  (Fig.  61). 

Distortion  of  the  kind  above  discussed  is  absent  with 
between-the-lens  shutters,  which  may  conceivably  be  im- 
proved in  efficiency  and  in  feasible  size.  If  so  they  would 
merit  serious  consideration  for  aerial  mapping. 

Methods  and  Apparatus  for  Testing  Shutter  Perform= 
ance. — With  a  focal-plane  shutter  the  desirable  qualities  in 
performance  are  three  in  number:  (1)  Adequate  speed  range, 
which  may  be  taken  as  from  5^  to  5^-  second  for  aerial  work, 

(2)  good   efficiency,   which   has  already  been  treated,  and 

(3)  uniformity  of  speed  during  its  travel  across  the  plate. 
Before  the  advent  of  aerial  photography  little   attention 
was  paid  to  speed  uniformity,  differences  of  50  per  cent,  in 
initial  and  final  speed  being  common  in  focal-plane  shutters, 
and  but  little  noticed  in  ordinary  landscape  work  because 
of  the  natural  variation  of  brightness  from  sky  to  ground. 
In  the  making  of  aerial  mosaic  maps  the  non-uniformity  of 
density  across  the  plate  results  in  a  most  offensive  series  of 
abrupt  changes  of  tone  at  the  junction  points  of  the  succes- 
sive prints  (Fig.  140),  an  effect  which  must  be  minimized 
by  manipulation  of  the  printing  light. 

Instruments  for  testing  the   speed   and   uniformity   of 
action  of  focal-plane  shutters  are  an  essential  part  of  any 
laboratory  for  developing  or  testing  photographic  apparatus 
and  some  simple  device  for  setting  and  checking  shutter 
speed  should  be  available  in  the  field.     Every  such  speed 


THE  SHUTTER 


77 


tester  must  contain  some  form  of  time  counting  element- 
pendulum,  tuning  fork  or  clockwork.  Elaborate  shutter 
testers,  suitable  for  determining  all  the  characteristics  of 
all  types  of  shutter,  have  been  developed  and  used  in  certain 
of  the  photographic  research  laboratories.  For  the  study  and 
setting  of  focal-plane  shutters  (whose  efficiency  need  not  be 
measured,  as  it  can  be  simply  calculated  from  linear  dimen- 
sions), the  following  simple  kinds  of  apparatus  are  adequate: 
Clock  dial  type  of  shutter  tester.  This  consists  essentially 
of  a  black  clock  dial  carrying  a  white  pointer  which  makes 
its  complete  revolution  in  one  second  or  less.  If  this  dial 
is  photographed  by  the  camera  under  test,  the  width  of  the 


FIG.  23. — Apparatus  for  testing  focal  plane  shutter  speed  throughout  the  travel  of  the  curtain. 

sector  traced  during  the  exposure  by  the  moving  pointer 
shows  the  time  interval.  If  the  dial  is  photographed  at 
several  points  on  the  plate — beginning,  middle  and  end  of 
the  shutter  travel — the  complete  characteristics  of  the 
shutter  can  be  determined. 

Interrupted  light  type  of  shutter  tester.  For  the  study  of 
uniformity  of  shutter  action  alone  the  apparatus  shown  in 
Fig.  23  may  be  employed.  A  is  a  high  intensity  light  source, 
such  as  an  arc  or  a  gas  filled  tungsten  lamp.  L  is  a  convex 
lens,  focussing  an  image  of  the  light  source  on  a  small  aper- 
ture in  the  screen  E.  D  is  a  sector  disc  which,  driven  by  the 
motor  M,  interrupts  the  transmitted  light  with  a  frequency 
determined  by  the  number  of  openings  of  the  sector  and  by 
the  speed  of  rotation,  which  must  be  measured  by  a  tacho- 


78          AIRPLANE  PHOTOGRAPHY 

meter.  The  light  diverging  from  the  aperture  in  E  falls 
upon  the  shutter  S9  which  for  this  test  is  reduced  to  a  narrow 
slit  of  one  millimeter  or  less.  Passing  through  the  shutter 
opening  the  light  falls  upon  the  photographic  plate  P. 
The  principle  is  simple:  If  the  light  is  uninterrupted,  the 
plate  P  is  exposed  at  all  points;  due  to  the  interruptions,  a 
series  of  parallel  lines  of  photographic  action  result,  and 


HOC 
1100 


V 

I      500J_ 
/S--    200 


ft "  10     11      ta.    13  4   14     fS     f6 


100  __J i!_L_J I 

o'    j    I    |    I    I   j  -J 

Start  Position   of  slit  in  centimeters  Finish 

FIG.  24. — Performance  of  Klopcic  shutter. 

their  distance  apart  gives  a  measure  of  the  speed  of  the 
shutter  at  any  chosen  point  in  its  travel.  A  performance 
curve  of  the  French  Klopcic  shutter  is  shown  in  Fig.  24. 
The  variation  in  speed  lies  over  a  range  of  two  to  one.  So 
serious  is  this  defect  in  these  shutters  that  diaframs  are 
sometimes  inserted  in  the  French  cameras  to  cut  off  part  of 
the  light  from  the  lens  on  the  most  exposed  end  of  the  plate. 
This  expedient  produces  uniformity  of  photographic  action, 


THE  SHUTTER 


79 


but  does  not  overcome  the  movement  of  the  image,  which  is 
one  of  the  chief  faults  of  excessive  exposure. 

A  more  complete  apparatus,  adapted  both  to  absolute 
speed  determinations  and  to  the  study  of  uniformity  of 
action,  is  that  worked  out  and  used  in  the  United  States 
Air  Service  (Fig.  25).  At  A  is  a  high  intensity  light  source, 
an  image  of  which  is  focussed  by  the  lens  Lx  upon  a  slit  E, 
in  front  of  which  stands  a  tuning  fork  T,  of  period  1024 
or  2048  per  second.  The  light  diverging  from  the  slit  is 
received  by  a  second  lens,  L2  which  is  arranged  either  to  focus 
the  slit  image  upon  the  shutter  curtain  or  to  render  the  rays 


L,    L 


LAMP  CONDENSERS    FORH'SLIT     LENS  CAMERA        SHUTTER  LENSES  DRUM 

FIG.  25. — Optical  system  of  shutter  tester  for  Air  Service,  U.  S.  Army. 

parallel,  so  that  an  entire  camera  may  be  inserted.  In  the 
latter  case  the  camera  lens  L3  serves  to  focus  the  slit  image 
on  the  curtain  C.  After  passing  through  the  curtain  aperture 
the  light  is  focussed  by  the  lens  L±  on  the  rotatable  drum  /), 
which  carries  a  strip  of  sensitive  film. 

The  operation  of  testing  a  shutter  consists  in  focussing 
the  slit  image  on  the  portion  of  the  shutter  whose  perform- 
ance is  required,  striking  the  tuning  fork  to  set  it  vibrating, 
rotating  the  drum  rapidly  and  setting  off  the  shutter. 
There  is  thus  obtained  on  the  sensitive  film  an  exposed  strip 
resembling  in  appearance  the  edge  of  a  saw,  the  number  of 
teeth  showing  the  time  interval  in  vibrations  of  the  tuning 
fork.  Three  exposures  usually  give  all  the  points  necessary 


80          AIRPLANE  PHOTOGRAPHY 

for  a  practical  knowledge  of  the  shutter's  uniformity  of 
action.  A  point  of  some  importance,  learned  from  numerous 
shutter  tests,  is  that  a  focal-plane  shutter  should  be  tested 
in  the  position  in  which  it  is  to  be  used.  Aerial  camera 
shutters  should  be  tested  in  the  horizontal  position. 

Types  of  Focal=plane  Shutters. — A  variety  of  means 
have  been  utilized  for  securing  the  necessary  variation  in 
speed  in  focal-plane  shutters.  Their  success  is  to  be  meas- 
ured by  the  actual  speed  range  and  by  the  uniformity  of 
speed  attained.  In  aerial  cameras  at  present  in  use  we  find 
variable  tension  of  the  curtain  spring,  the  aperture  being 
fixed;  variable  opening  with  fixed  tension;  multiple  curtain 
openings  with  fixed  spring  tension;  and  combinations  of  two 
or  all  of  these  methods  of  speed  control.  The  problem  of 
covering  the  aperture  during  the  operation  of  winding  up  or 
setting  the  shutter  has  led  to  further  elaborations  of  shutter 
mechanism.  These  take  the  form  of  lens  or  shutter  flaps,  auxil- 
iary curtains,  and  shutters  of  the  self-capping  type.  Shutters 
embodying  all  these  features  are  briefly  described  below. 

Representative  Shutters. — The  Folmer  variable  tension 
shutter  is  used  on  the  United  States  Air  Service  hand-held 
and  hand-operated  plate  camera  and  on  some  of  the  film 
cameras.  It  consists  of  a  fixed  aperture  curtain  wound  on 
a  curtain  roller  in  which  the  spring  can  be  set  to  various 
tensions,  numbered  1  to  10.  The  range  of  speeds  attainable 
is  at  best  about  three  to  one,  or  from  Y^-Q  to  3^  second, 
considerably  shorter  than  the  range  indicated  as  desirable. 
Its  uniformity  of  travel  is  variable  with  the  tension,  as  shown 
by  representative  performance  curves  in  Fig.  30.  Lacking 
any  self -capping  feature  the  shutter  is  provided  either  with 
an  auxiliary  curtain,  or  in  the  hand-held  camera  with  flaps 
in  front  of  the  lens,  opened  by  the  exposing  lever  before  the 
curtain  is  released  (Fig.  39).  This  shutter  is  made  a  re- 


THE  SHUTTER 


81 


movable  unit  in  the  18X24  centimeter  hand-operated  cam- 
era, but  is  built  into  the  hand-held  and  film  cameras. 

The  lea  shutter  used  on  the  standard  German  aerial 
cameras  is  a  good  example  of  the  multiple  slit  curtain 
(Fig.  26).  Four  fixed  aperture  slits  are  provided,  with  a  single 


FIG.  26. — Removable  four-slit  shutter  of  German  (lea)  camera,  showing  Saps. 

tension,  the  openings  roughly  in  the  ratio  1,  ^,  ^,  •§-,  which 
when  the  spring  tension  is  properly  adjusted  give  exposures 
°f  ^o>  T^o>  "3TT 5  TFO  second.  To  pass  from  one  exposure 
time  to  another  the  setting  milled  head  is  wound  up  to  suc- 
cessively higher  steps  or  else  exposed  one  or  more  times 
without  resetting,  depending  on  the  direction  it  is  desired 
to  go.  Capping  during  setting,  or  during  exposure,  in  order 
6 


82          AIRPLANE  PHOTOGRAPHY 

to  change  the  opening,  is  provided  for  by  a  pair  of  flaps  on 
the  shutter  unit,  which  open  into  the  camera  body.  The 
mechanical  work  on  these  shutters  is  of  excellent  quality, 
the  curtain  running  with  exceptional  smoothness.  Provision 
is  made  for  adjusting  the  tension  until  the  marked  speeds 
are  attained;  this  is  presumably  done  in  a  repair  laboratory 
to  which  the  shutter  only  need  be  sent,  as  it  is  a  removable 
unit.  Tests  made  on  one  of  these  shutters  wound  to  its 
highest  tension  are  shown  in  Fig.  30.  The  marked  speeds 
are  not  attained,  and  there  is  considerable  lack  of  uniform- 
ity from  start  to  finish  of  the  travel. 

L  camera  variable-aperture  shutter.  The  shutter  of  the  L 
type  camera  (Fig.  27)  is  representative  of  one  of  the  most 
primitive  methods  of  varying  aperture.  The  two  jaws  of 
the  slit  are  held  together  by  a  long  cord  passing  completely 
around  the  aperture,  fastened  permanently  at  one  end  and 
attached  at  its  other  end  by  a  sliding  clasp  or  saddle.  As 
this  saddle  is  forced  in  one  direction  the  slit  is  closed,  in  the 
opposite  direction  the  cord  becomes  slack,  and  after  the  shut- 
ter is  released  once  or  twice  the  slit  assumes  a  wider  opening. 
A  chronic  trouble  is  the  breaking  of  the  cords.  Its  opening 
can  be  changed  only  after  the  plate  magazine  is  removed. 

U.  S.  Air  Service  variable-aperture  shutter.  This  shutter 
is  incorporated  in  the  American  deRam  and  in  other  late 
American  cameras  (Fig.  28).  Its  characteristic  feature  is 
the  introduction  of  an  idler,  whose  distance  from  the  main 
curtain  roller  can  be  varied.  Tapes  whereby  the  following 
curtain  is  attached  to  the  spring  roller  pass  over  this  idler, 
and  by  changing  its  position  the  aperture  or  distance  between 
the  two  curtain  elements  is  altered  over  a  large  range. 
Tests  of  this  shutter  are  shown  in  Fig.  30.  A  speed  of  -£$ 
second  is  provided  for  by  a  slit  width  of  five  centimeters, 
and  the  highest  speed  is  fixed  only  by  the  practical  limit  of 


THE  SHUTTER 


83 


L 


FIG.  27. — "L"  type  camera  showing  open  negative  magazines  and  shutter  mechanism. 


84 


AIRPLANE  PHOTOGRAPHY 


approach  of  the  jaws.  Experiment  shows  great  uniformity 
of  rate  of  travel  to  be  attainable  by  combining  careful  choice 
of  spring  length  and  tension  with  good  workmanship  in  the 
mechanical  features.  Variable-aperture  fixed-tension  shut- 
ters have  a  definite  advantage  over  the  variable-tension 
type  in  that  they  can  utilize  for  all  speeds  that  tension  which 
gives  uniform  action.  The  capping  feature  of  this  shutter  is 
provided  in  the  American  deRam  by  flaps,  in  the  automatic 


IDLER 


FIG.  28. — Variable  aperture  curtain  developed  in  IT.  S.  Air  Service,  and  used  in  American   deRam, 
and  "K"  type  automatic  film  cameras. 

film  camera  by  an  auxiliary  curtain.    The  shutter  is  remov- 
able in  the  deRam,  but  built  into  the  other  camera. 

The  Klopcic  variable-tension,  variable-aperture,  self- 
capping  shutter  is  an  example  of  an  attempt  to  meet  all 
shutter  requirements  with  an  entirely  self-contained  mechan- 
ism. It  is  shown  diagrammatically  in  Fig.  29.  Tapes 
G1?  G2  are  used  to  connect  the  following  curtain  B 
directly  to  the  spring  roller  T,  at  a  fixed  distance,  while  the 
leading  curtain,  A,  may  be  slid  along  the  tapes  by  small  fric- 
tion buckles,  C19  C2,  auxiliary  springs  R^ ,  R2  serving  to  keep 
it  taut  in  any  position.  When  the  shutter  is  being  set  the 


THE  SHUTTER 


85 


buckles  are  arrested  against  stops  while  the  winding-up 
continues  for  what  is  to  be  the  following  half  of  the  curtain 
in  exposing.  When  released  the  curtain  moves  across 
with  an  aperture  fixed  by  the  point  of  setting  of  the  buckle 
stops.  At  the  end  of  the  travel  the  buckles  are  arrested  by 
other  stops,  while  the  following  portion  of  the  curtain  con- 


FIG.  29. — Mechanism  of  Klopcic  variable  aperture  self-capping  shutter. 

tinues  its  travel  to  the  end.  On  re-winding,  therefore,  the 
aperture  is  closed.  Variable  tension  as  well  as  variable 
aperture  is  provided,  although  little  used.  In  the  French 
cameras  a  lens  flap  is  also  inserted  behind  the  lens,  but  this 
is  not  needed  if  the  self-capping  feature  functions  properly. 
On  the  hand  cameras  this  flap  is  said  to  be  necessary  in 
order  to  prevent  a  curious  kind  of  accident:  if  the  camera 
is  held  on  the  knee,  pointing  upward,  an  image  of  the  sun 
may  be  formed  on  the  curtain  and  burn  a  hole  through  it. 


86 


AIRPLANE  PHOTOGRAPHY 


The  performance  of  the  French  shutter  in  respect  to 
uniformity  has  already  been  shown  in  Fig.  24.  It  leaves 
very  much  to  be  desired.  Besides  non-uniformity  of  action 
during  its  travel  it  exhibits  another  common  defect  of 
variable-tension  shutters,  namely,  the  curtain  must  be  re- 
leased several  times  after  a  change  of  tension  before  the  new 
speed  is  established  (Fig.  30,  tensions  5  and  5'). 


300 


ZOO 


START  MIDDLE  FINISH 

KLOPCIC  SHUTTER,  VARIABLE  APEKTUKC, 
VARIABLE  TENSION  TYPE. 


STAJST  MIDDLE  FINISH 

VARIABLE  TENSION,  FIXED  APE/STURE, 
USED  IN  HAND  OPERATED  /8x24  CM.  PLATE  CAMESA 


START  MIDDLE  FINISH 

FIXED  TENSION,  VARIABLE  APES  TUKE,  IDLER 
PATTERN   USED  IN  AMES/CAN  DE&AM. 


STA&T  MIDDLE:  FINISH 

FIXED  TENSION,  MULTIPLE  SLIT  CURTAIN,USED' 
IN  GERMAN  ICA    !3x/8  CM  CAMERA  . 


FIG.  30. — Performances  of  various  shutters  used  on  aerial  cameras.    Speeds  expressed  in  reciprocals 
of  fractional  parts  of  one  second. 

The  French  shutter  as  made  for  the  de  Maria  cameras 
is  a  removable  unit.  The  small  size  (13X18  cm.)  sets  by 
the  straight  pull  of  a  projecting  pin,  the  larger  (18  X24  cm.) 
by  winding  up  a  milled  head.  The  former  is  the  more  con- 
venient motion  for  an  aerial  camera.  Care  must  be  taken 
with  either  type  that  the  motion  of  setting  is  not  stopped 
when  the  first  resistance  is  encountered;  this  occurs  when 
the  tape  buckles  strike  their  stop  and  the  slit  begins  to  open. 


CHAPTER  VI 
PLATE-HOLDERS  AND  MAGAZINES 

In  the  earlier  days  of  airplane  photography  the  ordinary 
plate-holder  or  double  dark  slide  was  used  to  some  extent, 
but  it  is  ill-suited  to  the  purpose  because  of  the  considerable 
time  and  attention  required  for  its  operation.  It  has  never- 
theless the  merit  of  adding  little  to  the  length  of  the  camera, 
and  it  works  in  any  position.  For  these  reasons  it  has  re- 
mained in  occasional  use  for  the  taking  of  oblique  views 
with  long  focus  cameras  in  a  cramped  fuselage. 

Next  in  order  of  progress  rank  the  simple  box  magazines, 
for  holding  a  dozen,  eighteen  or  twenty-four  plates,  as  used 
in  the  English  C,  E,  and  L  type  cameras.  These  are  little 
more  than  boxes  with  sliding  lids  which  when  open  permit 
the  introduction  or  removal  of  the  plates.  Figs.  45  and  46 
illustrate  the  magazine  of  this  type  as  made  for  the  English 
C  and  E  cameras.  It  is  constructed  of  wood,  grooved  to  fit 
tracks  on  the  camera,  and  is  furnished  with  a  sliding  door  or 
lid  hinged  in  the  middle  to  fold  down  out  of  the  way  when 
open.  The  eighteen  plates  are  carried  in  metal  sheaths, 
both  to  provide  opaque  screens  between  them,  and  to  protect 
them  from  injury  in  the  mechanism  of  the  camera.  Fig.  27 
shows  the  all-metal  magazine  made  for  the  American  model 
L  camera.  This  differs  from  the  English  in  material  of  con- 
struction, plate  capacity  (24  instead  of  18)  and  manner  of 
operating  the  slide,  which  is  built  up  of  three  thicknesses  of 
phosphor  bronze  and  draws  out  through  metal  guides  bent 
into  semicircular  form.  A  snap  catch  holds  this  slide  at 
either  end  of  its  travel.  The  leather  strap  introduced  in  the 
American  model  for  carrying  and  handling  is  a  distinct 

87 


88 


AIRPLANE  PHOTOGRAPHY 


improvement.  These  magazines  contain  no  springs  or  other 
mechanism,  as  the  cameras  with  which  they  are  used  depend 
upon  the  action  of  gravity  for  emptying  the  upper  (feeding) 
magazine,  and  filling  the  lower  (receiving)  one. 

Next  in  order  of  complexity  may  be  ranked  the  bag 
magazine  (Figs.  31  and  44).  In  this  the  exposed  plate  is 
pulled  out  of  the  magazine  proper  by  a  metal  slide  or  rod 
into  a  leather  bag.  The  rod  is  then  pushed  back,  the  plate 
in  its  metal  sheath  is  grasped  through  the  leather  bag,  lifted 


FIG.  31.— Aerial  hand  camera  (U.  S.  type  A-2). 

to  the  back  of  the  magazine,  and  forced  in  behind  the  other 
plates.  The  number  of  plates  exposed  is  indicated  either  by 
numbers  on  the  backs  of  the  sheaths,  visible  through  a  red 
glazed  opening  in  the  back,  or  else  by  a  counter  actuated 
by  the  metal  slide  rod.  Usually  twelve  are  carried  in  a 
magazine.  For  aerial  work  the  common  design  of  this 
magazine  as  used  for  ground  work  must  be  modified  by 
providing  extra  large  easily  grasped  hooks  both  on  the  draw 
rod  and  on  the  dark  slide,  which  must  be  drawn  before 


PLATE-HOLDERS  89 

making  the  first  exposure  and  replaced  after  the  last.  The 
small  rings  and  grips  of  the  standard  commercial  magazine 
are  almost  impossible  to  handle  through  heavy  gloves. 

The  next  type  of  magazine  is  represented  by  three 
designs,  the  Gaumont  and  de Maria,  used  very  generally  by 
the  French  during  the  war,  and  the  Ernemann,  used  almost 
universally  in  the  German  air  service  (Figs.  32,  40  and  42). 
In  all  of  these  the  operation  of  plate  changing  is  the  same: 
the  end  of  the  magazine  is  pulled  out  and  thrust  back,  a  more 
simple  operation  than  the  bag  manipulation  just  described. 
The  internal  workings  are  different  according  to  size.  In 
the  smaller  French  magazines  (13X18  cm.)  the  camera  is 
first  pointed  upward,  all  the  plates  are  drawn  out  except 
the  one  to  be  changed,  and  this,  with  the  aid  of  springs,  drops 
to  the  bottom,  after  which  the  other  plates  push  back  over 
it.  The  plates  pull  out  in  the  direction  of  their  long  dimen- 
sion. In  the  larger  French  magazine  (18X24  cm.)  only  the 
exposed  plate  pulls  out.  The  pull  is  in  the  direction  of  the 
shorter  dimension  of  the  plate,  which  is  lifted  up  by  heavy 
springs  and  slides  back  over  the  top  of  the  pile.  In  the 
Ernemann  magazine  only  six  plates  are  carried,  which  there 
is  good  reason  to  believe  represent  the  maximum  feasible 
number,  judging  by  the  reports  of  jambs  and  breakages  in 
the  twelve-plate  French  magazines.  In  all  of  these  magazines 
laminated  wood  slides  pull  out  and  in  at  each  operation, 
and  while  satisfactory  if  made  and  operated  in  one  climate, 
experience  indicates  that  if  made  in  America  and  sent  abroad 
swelling  of  the  wood  may  be  expected  to  prevent  their 
successful  operation. 

Alternative  forms  of  magazine,  somewhat  more  practical 
from  the  standpoint  of  manufacture  and  export,  are  several 
designs  embodying  two  compartments  (Fig.  32) .  In  the  most 
simple  of  these  the  plates  are  moved,  immediately  before 


90 


AIRPLANE  PHOTOGRAPHY 


or  after  exposing,  from  the  unexposed  to  the  exposed  side. 
Illustrative  of  this  type  are  the  Folmer  designs,  in  which  the 
to-and-fro  motion  is  imparted  by  a  rack  geared  to  a  pinion 


deft  or/a 


.Piserini  %  Man  dint 


UJ 


Hutton 


Bou  long 


er 


Be  Hi  en  i 


FIG.  32.  —  Various  plate  magazines  used  on  aerial  cameras. 

actuated  either  by  a  lever,  in  the  hand  camera,  or  by  the 
power  drive,  in  the  automatic  design  (Figs.  33  and  53).  An- 
other illustration  is  afforded  bv  the  Piserini  and  Mondini 


PLATE-HOLDERS 


91 


FIG.  33. — U.  S.  Air  Service  hand  camera,  with  two-compartment  magazine. 


FIG.  34.— Film  type  hand  camera 


magazine,  in  which  the  operation  of  changing  is  performed 
by  a  back-and-forth  motion  of  a  hand-grip,  which  also  sets 
the  camera  shutter  (Fig.  47). 


92          AIRPLANE  PHOTOGRAPHY 

In  these  magazines  the  center  of  gravity  changes  as  the 
exposed  plates  are  moved  over,  and  only  half  the  inside 
space  is  occupied  with  plates.  These  objections  are  over- 
come in  the  Chassel  form,  where  both  compartments  are 
always  full.  Transfer  of  the  bottom  exposed  plate  from  one 
compartment  to  the  other  is  compensated  for  by  the  simul- 
taneous shift  of  the  top  plate  in  the  receiving  compartment, 
to  the  feeding  side.  In  a  modification  of  this  idea  by  Ruttan 
the  exposing  position  is  when  the  plates  are  half-way  through 
the  shifting  process,  whereby  the  magazine  may  be  symmet- 
rically mounted  on  the  camera  body. 


FIG.  35. — Apparatus  for  straightening  plate  sheaths. 

Other  more  complicated  magazines  have  been  designed, 
some  of  which  are  shown  in  the  diagrammatic  ensembles  of 
Figs.  32  and  48.  In  the  Jacquelin,  the  main  body  of  plates 
is  raised  while  the  bottom  (exposed)  plate  is  folded  against 
the  side.  The  main  body  of  plates  then  drops  back  to  place, 
the  exposed  plate  is  carried  on  upward  and  folds  down  on 
the  back  of  the  pile.  The  Bellieni  magazine  system  uses 
three,  a  central  feeding  one  and  two  below  for  receiving, 
one  on  each  side  of  the  camera  body.  These  were  made 
solely  for  attachment  to  captured  German  cameras.  In 
the  Fournieux  magazine  the  plates  are  carried  in  an  interior 


PLATE-HOLDERS 


93 


rotating  box.  The  plate  to  be  exposed  is  dropped  off  the 
front  of  the  pile,  down  to  the  focal  plane,  and  after  exposure 
is  picked  up  and  placed  at  the  back  of  the  pile,  which  has 
turned  over  in  the  meanwhile.  The  deRam  rotating  maga- 
zine is  described  in  connection  with  the  camera  of  which  it 
is  an  essential  part  (Fig.  52). 


FIG.  36. — Training  plane  equipped  for  photography,  showing  "L"    camera    in   floor    mount  and 
magazine  rack  forward  of  the  observer. 

For  the  protect  ion  of  the  plates  during  their  manipulation, 
and  in  the  camera,  all  plate  magazines  thus  far  developed 
carry  them  in  thin  metal  sheaths.  These  add  greatly  both 
to  the  weight  and  to  the  time  necessary  to  handle  the  plates, 
but  no  means  have  as  yet  been  found  for  dispensing  with 


94          AIRPLANE  PHOTOGRAPHY 

them.  Fig.  35  shows  a  representative  sheath  or  septum,  as 
used  in  the  L  camera.  On  three  sides  the  edge  is  bent  up 
and  turned  over,  forming  a  ledge  for  the  plate  to  press 
against.  The  fourth  side  is  left  open  for  inserting  the  plate, 
which  is  then  held  in  by  a  small  upward  projecting  lip,  and 
kept  close  against  the  ledges  by  narrow  springs  at  the  sides. 
To  insert  or  remove  the  plate  the  projecting  lip  is  depressed, 
either  by  springing  the  sheath  by  pressure  from  the  sides 
or  by  using  an  appropriate  tool. 

Care  of  sheaths.  Unless  systematically  taken  care  of, 
plate  sheaths  become  bent  or  dented  They  are  then  a 
menace  to  camera  operation,  catching  or  jamming  in  the 
plate  changing  process,  breaking  plates  and  damaging 
camera  mechanisms.  In  order  to  maintain  them  flat  and 
true,  steel  forms  are  necessary  on  which  the  sheaths  may  be 
hammered  to  shape  with  a  mallet  (Fig.  35). 

Magazine  racks.  Reconnaissance  and  mapping  call  for 
a  number  of  exposures  much  greater  than  the  capacity  of 
one  12,  18,  or  24  plate  magazine.  Additional  magazines 
must  therefore  be  carried.  These  should  be  in  racks  con- 
venient to  the  observer  (Fig.  36),  securely  held  yet  capable 
of  quick  removal  and  insertion.  In  the  rack  designed  to 
carry  two  of  the  metal  magazines  for  the  American  L  Camera, 
the  magazines  slide  into  loose  grooves  formed  by  a  metal  lip. 
They  are  prevented  from  slipping  out  by  a  spring  catch, 
past  which  they  slide  when  inserted  but  which  is  instantly 
thrown  aside  by  pressure  of  the  thumb  as  the  hand  grasps 
the  magazine  handle  for  removal. 


CHAPTER  VII 
HAND-HELD  CAMERAS  FOR  AERIAL  WORK 

Field  of  Use. — The  first  cameras  to  be  used  for  aerial 
photography  were  hand-held  ones  of  ordinary  commercial 
types.  Indeed  the  idea  is  still  prevalent  that  to  obtain 
aerial  photographs  the  aviator  merely  leans  over  the  side 
with  the  folding  pocket  camera  of  the  department  store 
show  window  and  presses  the  button.  But  the  Great  War 
had  not  lasted  long  before  the  ordinary  bellows  focussing 
hand  camera  was  replaced  by  the  rigid-body  fixed-focus 
form,  equipped  with  handles  or  pistol  grip  for  better  holding 
in  the  high  wind  made  by  the  plane's  progress  through  the" 
air.  Even  this  phase  of  aerial  photography  was  compara- 
tively short-lived.  The  need  for  cameras  of  great  focal 
length,  and  the  need  for  apparatus  demanding  the  minimum 
of  the  pilot's  or  observer's  attention,  both  tended  to  relegate 
hand-held  cameras  to  second  place,  so  that  they  were  com- 
paratively little  used  in  the  later  periods  of  the  war. 

Yet  for  certain  purposes  they  have  great  value.  They 
can  be  used  in  any  plane  for  taking  oblique  views,  and  for 
taking  verticals,  in  any  plane  in  which  an  opening  for  unob- 
structed view  can  be  made  in  the  floor  of  the  observer's 
cockpit.  They  can  be  quickly  pointed  in  any  desired 
direction,  thus  reducing  to  a  minimum  the  necessary  ma- 
neuvering of  the  plane,  a  real  advantage  when  under  attack 
by  "Archies  "  or  in  working  under  adverse  weather  conditions. 

For  peace-time  mapping  work  the  hand-held  camera, 
when  equipped  with  spirit-levels  on  top,  and  when  worked 
by  a  skilful  operator,  possesses  some  advantages  over  any- 
thing short  of  an  automatically  stabilized  camera.  For 

95 


96          AIRPLANE  PHOTOGRAPHY 

experimental  testing  of  plates,  filters  and  various  acces- 
sories, the  ready  accessibility  of  all  its  parts  makes  the  hand- 
held camera  the  easiest  and  most  satisfactory  of  instruments. 

The  limitations  of  the  hand-held  camera  lie  in  its  necessary 
restriction  to  small  plate  sizes  and  short  focal  lengths,  and 
in  the  fact  that  it  must  occupy  the  entire  attention  of  the 
observer  while  pictures  are  being  taken — the  latter  a  serious 
objection  only  in  war-time. 

Essential  Characteristics. — In  addition  to  the  general 
requirements  as  to  lens,  shutter  and  magazine,  common  to 
all  aerial  cameras,  the  hand  camera  must  meet  the  special 
problems  introduced  by  holding  in  the  hands,  especially  over 
the  top  of  the  plane's  cockpit.  An  exceptionally  good  system 
of  handles  or  grips  must  be  provided  whereby  the  camera 
can  be  pointed  when  pictures  are  taken,  and  held  while 
plates  are  being  changed  and  the  shutter  set.  The  weight 
and  balance  of  the  camera  must  be  correct  within  narrow 
limits;  the  wind  resistance  must  be  as  small  as  possible;  the 
shutter  release  must  be  arranged  so  as  to  give  no  jerk  or  tilt 
to  the  camera  in  exposing. 

As  to  the  method  of  holding  the  camera,  a  favorite  at 
first  among  military  men  was  the  pistol  grip,  with  a  trigger 
shutter  release  (Fig.  37).  Because  of  the  size  and  weight 
of  the  camera  the  pistol  grip  alone  was  an  inadequate  means 
of  support  and  additional  handles  on  the  side  or  bottom  had 
to  be  provided  for  the  left  hand.  Small  (8X12  cm.)  pistol 
grip  cameras  were  used  to  some  extent  by  the  Germans 
(Fig.  42),  and  a  number  of  4X5  inch  experimental  cameras 
of  this  type  were  built  for  the  American  Air  Service  (Fig.  37). 
But  the  grasp  obtained  with  such  a  design  is  not  so  good  as  is 
obtained  with  handles  on  each  side  or  with  flat  straps  to  go 
over  the  hands.  The  camera  balances  best  with  the  handles 
in  the  plane  of  the  center  of  gravity.  As  to  weight,  no  set 


HAND-HELD  CAMERAS  97 

rules  are  laid  down,  but  experience  has  shown  that  a  fairly 
heavy  camera — as  heavy  as  is  convenient  to  handle — will 
hold  steadier  than  a  light  one.  The  American  4X5  inch 
cameras  described  below  weigh  with  their  magazines  in  the 
neighborhood  of  twelve  pounds. 


FIG.  37. — Pistol-grip  aerial  hand  camera. 

Representative  Types  of  Hand=held  Cameras. — French 
and  German  hand-held  cameras  are  essentially  smaller 
editions  of  their  standard  long-focus  cameras,  and  a  descrip- 
tion of  them  will  apply  to  a  considerable  extent  to  the  large 

7 


98 


AIRPLANE  PHOTOGRAPHY 


cameras  to  be  discussed  in  a  later  chapter.  The  English  and 
American  hand-held  cameras  are  generally  quite  different  in 
type  from  the  large  ones,  which  are  used  attached  to  the  plane. 
The  French  hand-held  camera  uses  13X18  centimeter 
plates,  carried  in  a  deMaria  magazine,  and  has  a  lens  of  26 
centimeters  focus.  The  shutter  is  the  Klopcic  self-capping 


FIG.  38. — Diagram  of  French  (deMaria)  26  cm.  focus  hand  camera,  using  13x18  cm.  plates. 

type  already  described,  and  is  removable.  The  camera  body, 
built  of  sheet  aluminum,  takes  a  pyramidal  shape.  In  Fig. 
38,  A  is  the  shutter  release  and  B  the  rectangular  sight,  of 
which  C  is  the  rear  or  eye  sight.  The  complete  sight  may  be 
placed  either  on  the  top  or  on  the  bottom  of  the  camera. 
At  D  are  the  handles,  sloping  forward  from  top  to  bottom; 
E  is  a  catch  for  holding  the  magazine;  Fis  a  door  for  reaching 
the  back  of  the  lens  and  the  lens  flap;  G  is  a  snap  clasp  for 


HAND-HELD   CAMERAS  99 

holding  the  front  door  of  the  camera  closed;  H  is  a  ring  for 
attaching  a  strap  to  .go  around  the  observer's  neck;  /  is  the 
lever  which  opens  the  flap  behind  the  lens  and  releases  the 
focal-plane  shutter;  J  is  a  snap  catch  for  holding  the  front 
door  of  the  camera  open. 

The  operations  with  this  camera  are  three  in  number. 
Starting  immediately  after  the  exposure,  the  camera  is 
pointed  lens  upward  and  the  plate  changed  by  pulling  the 
inner  body  of  the  magazine  out  and  then  in;  next  the  shutter 
is  set;  then  the  camera  is  pointed,  and  finally  exposed  by  a 
gentle  pull  on  the  exposing  lever. 

The  English  hand-held  camera  (Fig.  186);  This  differs 
from  the  French  in  the  size  of  plate  (4X5  inch),  in  the  shape 
of  the  camera  body,  which  is  circular,  and  in  the  type  of 
shutter,  which  is  fixed-tension  variable-opening.  In  the 
longer  focus  camera  (10  to  12  inch)  the  shutter  is  self -capping, 
and  the  aperture  is  controlled  by  a  thumb-screw  at  the  side. 
In  the  smaller  (6  inch)  a  lens  flap  is  provided  in  front  of  the 
lens  and  the  shutter  aperture  is  varied  by  a  sliding  saddle 
and  cord.  The  handles  of  the  camera  are  placed  vertical, 
instead  of  sloping  as  in  the  French.  The  shutter  is  released 
by  a  thumb-actuated  lever.  Double  dark  slides  are  used,  as 
the  multiple  plate  magazine  has  not  found  favor  in  the 
English  service. 

'The  German  hand-held  camera  (Fig.  42).  The  German 
hand-held  camera  is,  like  their  whole  series,  built  of  canvas- 
covered  wood,  the  body  having  an  octagonal  cross-section. 
It  is  equipped  with  the  lea  shutter  and  uses  the  Ernemann 
six  plate  (13X18  cm.)  magazine.  The  excellent  system  of 
grips  by  which  the  camera  is  held  and  pointed  is  an  especially 
commendable  feature.  On  the  right-hand  side  is  a  handle 
similar  to  the  French  type,  but  carefully  shaped  to  fit  the 
hand.  The  left-hand  grip  consists  of  a  long,  rounded  block  of 


100 


AIRPLANE  PHOTOGRAPHY 


wood  running  diagonally  from  top  to  bottom  of  'the  side, 
with  a  deep  groove  on  the  forward  side  for  the  finger  tips, 
while  over  the  hand  is  stretched  a  leather  strap,  the  whole 
aim  being  to  provide  an  absolutely  sure  and  comfortable 
hold  on  the  camera  during  the  plate  changing  and  shutter 
setting  operations. 


FIG.  39. — Front  view  of  U.  S.  aerial  hand  camera,  showing  lens   flaps   partly   open,   and  details  of 

tube  sight. 

United  States  Air  Service  hand  cameras.  The  hand  camera 
developed  for  the  United  States  Air  Service  and  manufac- 
tured by  the  Eastman  Kodak  Co.  is  made  in  three  models, 
using  the  bag  magazine,  a  two-compartment  magazine,  and 
roll  film,  respectively.  The  shutter  is  of  the  fixed  (one  or  two) 
aperture  variable  tension  type,  built  into  the  camera.  A 


HAND-HELD   CAMERAS  101. 

distinctive  feature  is  the  double  lens  flap,  in  front  of  the  lens 
actuated  by  the  thumb  pressure  shutter  release  (Fig.  39). 
In  the  bag  magazine  camera  the  shutter  is  set,  as  a  separate 
operation,  by  a  wing  handle,  and  a  similar  handle  controls 
the  tension  adjustment.  In  the  two-compartment  type 
(Fig.  33)  the  shutter  wind-up  is  geared  to  the  plate  changing 
lever,  so  that  but  one  operation  is  necessary  to  prepare  the 
camera  for  exposure.  In  the  film  type  (Fig.  34)  a  single  lever 
motion  sets  the  shutter  and  winds  up  the  film  ready  for 
the  next  exposure.  After  the  last  exposure  of  all  the  film  is 
wound  backward  on  its  own  (feeding)  roller  before  remov- 
ing from  the  camera.  The  film  is  held  flat  by  a  closely  fit- 
ting metal  plate  behind,  and  by  guides  at  the  edges  in  front, 
an  arrangement  which  with  small  sizes  works  fairly  well 
although  the  exquisite  sharpness  of  focus  attainable  with 
plates  is  not  to  be  expected.  The  saving  in  weight  made 
possible  by  the  use  of  film  in  place  of  plates  in  metal 
sheaths  is  about  three  pounds  per  dozen  exposures. 

In  all  these  cameras  the  sight — a  tube  with  front  and  back 
cross  wires — is  placed  at  the  bottom.  This  position  has  been 
found  the  most  convenient  for  airplane  work,  as  it  neces- 
sitates the  observer  raising  himself  but  little  above  the  cock- 
pit, a  matter  of  prime  importance  when  the  tremendous 
drive  of  the  wind  is  taken  into  account. 


CHAPTER  VIII 
NON-AUTOMATIC  AERIAL  PLATE  CAMERAS 

The  ideal  of  every  military  photographic  service  has  been 
an  automatic  or  at  least  a  semi-automatic  camera,  in  order 
to  reduce  the  observer's  work  to  a  minimum.  Yet  as  a 
matter  of  fact  almost  all  the  aerial  photography  of  the  Great 
War  was  done  with  entirely  hand-operated  cameras.  The 
primary  reason  for  this  was  that  no  entirely  satisfactory 
automatic  cameras  were  developed,  cameras  at  once  simple 
to  install  and  reliable  when  operated.  Even  the  propeller- 
drive  semi-automatic  L  type  of  the  British  Air  Service  was 
very  commonly  operated  by  hand,  for  many  of  the  pilots 
and  observers  regarded  the  propeller  merely  as  another  part 
to  go  wrong. 

Any  automatic  mechanism  in  the  airplane  must  work 
well  in  spite  of  vibration,  three  dimensional  movements, 
and  great  range  of  temperature.  The  requirements  were 
well  recognized  when  the  war  closed,  but  had  not  yet  been 
met.  Careful  study  of  the  conditions  and  needs  by  compe- 
tent designers  of  automatic  machinery  may  be  expected  to 
result  at  an  early  date  in  reliable  cameras  of  the  automatic 
type,  but  the  description  below  of  hand-operated  cameras 
really  covers  practically  all  the  cameras  found  satisfactory 
in  actual  warfare. 

General  Characteristics  of  Hand=operated  Cameras.— 
As  distinguished  from  the  hand-held  cameras  the  larger 
hand-operated  cameras  are  characterized  by  the  greater 
focal  length  of  their  lenses,  the  size  of  plate  employed,  and 
the  manner  of  holding — by  some  form  of  anti-vibration 
mounting  attached  directly  to  the  fuselage. 
102 


NON-AUTOMATIC  CAMER'AS      103 

Except  for  the  early  English  C  and  E  type  cameras  which 
called  for  10  inch  lenses  and  4X5  inch  plates,  the  general 
practice  at  the  close  of  the  war  by  agreement  between  the 
French,  English  and  American  Air  Services,  was  for  the  use 
of  18X24  centimeter  plates  and  for  lenses  with  focal  lengths 
of  approximately  25,  50  and  120  centimeters.  The  English 
also  made  use  of  a  14  inch  (35  centimeter)  lens,  and  never 
made  a  regular  practice  of  anything  larger  than  50  centi- 
meters. The  Germans  and  Italians  restricted  themselves 
to  the  13X18  centimeter  size  of  plate,  while  a  lens  of  70 
centimeters  focal  length  was  standardized  with  the  Germans, 
in  addition  to  the  25,  50,  and  120  centimeter. 

The  particular  focal  length  was  determined  by  the  nature 
of  the  photographic  mission.  Where  large  areas  were  to  be 
covered  at  low  altitudes  or  without  the  demand  for  exquisite 
detail,  the  shorter  focus  lenses  suffice.  The  most  commonly 
used  lens  in  the  French  Service  was  the  50  centimeter,  while 
the  120  was  employed  when  high  flying  was  necessary  or 
when  minute  detail  was  required.  As  already  mentioned, 
the  common  practice  was  to  keep  cameras  of  all  focal  lengths 
available,  but  the  ideal  at  the  close  of  the  war  was  to  have 
the  camera  nose  and  lens  a  detachable  unit,  so  that  any  focal 
length  desired  could  be  secured  with  the  same  camera  body. 

The  standard  French  camera.  The  hand-held  form  of 
French  camera  has  already  been  described.  The  cameras 
for  larger  plate  sizes  and  longer  focus  lenses  differ  only  in 
the  addition  of  a  Bowden-wire  distance  release  for  the 
shutter  and  in  the  use  of  the  Gaumont  magazine  which 
operates  without  the  necessity  of  pointing  the  exposed  side 
of  the  magazine  upward.  Fig.  40  illustrates  the  50  centi- 
meter camera,  and  Fig.  41  the  120. 

The  German  lea  cameras.  These  are  larger  editions  of 
the  light  wood  hand  camera  already  described,  but  with  the 


104        AIRPLANE  PHOTOGRAPHY 


r 


FIG.  40. — 50  centimeter  deMaria  hand  operated  camera  on  tennis  ball  mounting. 


NON-AUTOMATIC  CAMERAS      105 


»2 


FIG.  41. — 120  centimeter  deMaria  camera. 


106        AIRPLANE  PHOTOGRAPHY 

addition  of  a  Bowden-wire  shutter  release.  The  body  of  the 
larger  cameras  carries  a  distinctive  feature  in  the  distance 
control  of  the  lens  diafram,  worked  by  means  of  a  lever 
which  actuates  racks,  pinions  and  connecting  rods  leading 
to  the  lens.  On  the  side  of  the  camera  body  a  shallow  box 


FIG.  42. — German  aerial  cameras. 

is  provided  for  carrying  the  color  filter  in  its  bayonet  joint 
mount  to  fit  on  the  lens  (Figs.  42  and  43). 

The  hand-operated  bag -magazine  camera  of  the  United 
States  Air  Service  (Type  M)  is  similar  to  the  small  hand- 
held camera,  but  differs  in  three  respects:  a  removable 
shutter  (of  the  variable-tension  fixed-aperture  type)  embody- 
ing an  auxiliary  curtain  for  capping  during  the  setting  opera- 


NON-AUTOMATIC  CAMERAS      107 


FIG.  43. — Diagram  of  German  50  centimeter  camera. 


108 


AIRPLANE  PHOTOGRAPHY 


tion;  a  Bowden-wire  shutter  release;  and  the  employment 
of  a  set  of  standard  interchangeable  cones  to  hold  lenses  of 
several  focal  lengths.  The  20  inch  and  10  inch  cones  are 


FIG.  44. — U.  S.  hand-operated  aerial  camera  (type  M)  with  10  and  20  inch  cones. 

shown  in  Fig.  44.  The  operation  of  this  camera  is  similar 
to  the  French  standard  cameras,  but  not  so  simple  because  of 
the  number  of  motions  required  in  manipulating  the  bag. 


NON-AUTOMATIC  CAMERAS      109 

Its  chief  objection  for  war  work  lies  in  fact  in  the  magazine, 
which  should  be  superseded  by  a  two-compartment  or  other 
satisfactory  type  of  plate  changing  chamber.  The  camera 
alone,  with  20  inch  cone,  weighs  approximately  40  pounds;  the 
loaded  magazine,  with  its  plates  in  metal  sheaths,  15  pounds. 
The  English  C  and  E  type  cameras.  The  C  and  E  type 
cameras  have  now  chiefly  an  historic  interest.  They  were 


FIG.  45. — English  C  type  aerial  camera. 

the  first  used  in  the  English  service,  fixed  to  the  fuselage, 
and  were  later  used  in  training  work  in  England  and  in  the 
United  States.  They  were  never  built  for  plates  larger  than 
4X5  inch  nor  for  lenses  of  more  than  12  inch  focus,  a  limita- 
tion set  by  the  lenses  available  at  the  time  of  their  design. 
In  several  respects  the  mode  of  operation  of  the  two 
types  is  the  same.  The  unexposed  plates  are  held  in  a  maga- 
zine lying  above  the  camera,  in  the  axis  of  the  lens  (Fig.  32). 


110 


AIRPLANE  PHOTOGRAPHY 


After  exposure  the  bottom  plate  is  carried  to  one  side  and 
allowed  to  fall  by  the  action  of  gravity  into  the  receiving 
magazine.  In  the  C  type  (Fig.  45)  an  opaque  slide  is  drawn 
between  the  lens  and  the  (variable-opening)  shutter  during 
the  setting  operation.  During  the  exposure  period  this  slide 
projects  into  a  compartment  on  the  opposite  side  of  the 


Fio.  46. — English  type  "E"  hand-operated  plate  camera. 


camera   from   the 
camera  mechanism 
46),  a  flap  over  the 
the  sliding  screen, 
width  of  two  plates 
by  a  handle  on  top 
is  made  for  distance 


receiving  magazine,  thus  making  the 
three  plates  wide.  In  the  E  type  (Fig. 
lens  makes  it  possible  to  dispense  with 
and  reduces  the  camera  to  about  the 
.  In  the  C  type  the  plates  are  changed 
of  the  camera;  in  the  E  type  provision 
control  by  cords,  and  for  shutter  release 


NON-AUTOMATIC  CAMERAS      111 


by  a  Bowden  wire.  In  both  cameras  the  operation  of  plate 
changing  also  sets  the  shutter,  a  definite  advance  over  the 
two  preparatory  motions  in  the  French  apparatus.  The  C 
type  was  constructed  of  wood,  the  E  of  metal. 


FIG.  47. — Italian   (Piserini  and  Mondini)  two  compartment  magazine  hand-operated  camera. 

Italian  two-compartment  magazine  camera.  A  camera 
designed  by  Piserini  and  Mondini  was  used  to  some  extent 
by  the  Italian  service  toward  the  close  of  the  war  (Fig.  47). 
This  has  the  desirable  feature  just  noted  in  the  C  and  E 
cameras:  the  operations  of  plate  changing  and  shutter 
setting  are  performed  in  a  single  motion.  Unlike  those 
cameras,  however,  the  plates  are  changed  from  one  compart- 


AIRPLANE  PHOTOGRAPHY 

ment  to  another  of  the  magazine  already  described,  without 
dependence  on  gravity,  by  an  entirely  positive  shifting 
action.  The  setting  of  the  self-capping  focal-plane  shutter 
is  accomplished  by  a  projecting  finger  engaging  the  shutter 
mechanism.  Cameras  of  this  general  type,  built  for  18  by 
24  centimeter  plates,  with  interchangeable  lens  cones, 
removable  shutters,  and  preferably  magazines  in  which  the 
center  of  gravity  does  not  shift  as  the  plates  are  changed, 
represent  the  next  step  in  advance  of  the  French  practice, 
and  may  indeed  prove  all  that  is  necessary  or  desirable  in 
camera  complexity  for  peace-time  photography  from  the  air. 

The  standard  Italian  camera  and  similar  types.  The 
camera  (Lamperti)  which  the  Italian  Air  Service  used 
almost  exclusively  during  the  war  exemplifies  a  type  quite 
different  from  anything  as  yet  described  (Figs.  48  and  49). 
Plates  to  the  number  of  twenty-four  (13  X18  cm.)  are  loaded 
into  a  chamber  at  the  top  of  the  camera.  Each  plate  is  held 
in  a  septum  furnished  with  projecting  lugs  at  one  end.  A 
lever  acting  through  a  Bowden  wire,  exposes  the  bottom 
plate,  which  then  swings  downward  about  these  lugs  as 
pivots,  and  is  forced  by  a  pair  of  fingers  into  a  compartment 
at  the  side.  The  between-the-lens  shutter  has  a  single  speed 
of  1/150  second,  and  variation  of  exposure  is  achieved  by 
altering  the  lens  aperture. 

The  great  advantage  of  this  camera  is  its  simplicity,  a 
single  motion  performing  all  the  operations.  Its  disadvan- 
tages are  its  dependence  on  gravity  for  operation,  its  be- 
tween-the-lens shutter,  the  limitation  set  to  the  number  of 
exposures,  and  the  necessity  for  removing  the  whole  camera 
to  take  out  the  plates  for  developing.  In  actual  practice 
the  camera  has  worked  out  well.  The  better  light  found  in 
the  Italian  as  contrasted  with  the  northern  theatre  of  war 
makes  the  between-the-lens  shutter  at  high  speed  adequate, 


NON-AUTOMATIC  CAMERAS      113 

X? 


Shutter— ' 


Avbry 


Unexposed 


Exposed 


Unexposed 

k. 

Exposed 

=Q- 

Lam  pert- i 


Premaero 


fournieux 

FIG.  48. — Various  plate  changing  devices. 


114        AIRPLANE  PHOTOGRAPHY 

while  the  limitation  to  the  number  of  exposures  has  been 
met  by  carrying  several  complete  cameras  in  each  plane. 
Because  of  the  Bowden-wire  operation  these  cameras  need 
not  be  accessible  to  the  observer  or  pilot,  so  that  the  practice 


FIG.  49. — Italian  (Lamperti)  single-motion  plate  camera,  on  anti-vibration  tray. 

of  carrying  them  in  single-seaters  was  common.  Attempts 
at  standardization  of  Allied  practice  through  the  adoption 
of  standard  lens  cones  were,  of  course,  out  of  the  question 
with  this  camera.  With  its  limitations  of  shutter  efficiency 


NON-AUTOMATIC  CAMERAS      115 

and  plate  size  it  is  doubtful  whether  it  would  have  been 
satisfactory  outside  the  service  for  which  it  was  developed. 
The  limitations  set  by  the  between-the-lens  shutter  in 
this  type  have  been  overcome  in  an  experimental  camera 
along  similar  lines  made  by  the  Premo  Works  of  the  Eastman 
Kodak  Company,  and  in  the  French  Aubry  model  (Fig.  48). 
These  employ  focal-plane  shutters  which  swing  out  of  the 
way  and  are  set  as  the  exposed  plate  swings  or  drops  to  the 
receiving  chamber.  The  dependence  on  gravity  in  this  type 
could  doubtless  be  avoided  by  positive  finger  mechanisms. 
If  so,  the  resultant  cameras,  set  and  exposed  by  a  single 
motion,  would  acquire  a  highly  desirable  simplicity  of  opera- 
tion. They  would  have  peculiar  merit  because  of  the  very 
short  interval  required  between  exposures — a  characteristic 
needed  for  making  low  stereo-oblique  views.  The  cameras 
just  mentioned  have,  however,  departed  far  in  form  from  the 
lines  of  standardized  practice  and  have  not  been  followed  up. 


CHAPTER  IX 
SEMI-AUTOMATIC  AERIAL  PLATE  CAMERAS 

In  the  hand-operated  camera  the  limit  to  progress  is  set 
when  the  number  of  operations  is  reduced  to  a  minimum. 
In  cameras  using  the  larger  sizes  of  plates  a  reduction  in  the 
number  of  operations  almost  inevitably  results  in  inflicting 
considerable  muscular  labor  upon  the  operator.  Furthermore, 
distance  operation  becomes  difficult  to  arrange  for,  because 
the  common  reliance — the  Bowden  wire — is  unfitted  for 
heavy  loads.  Consequently,  for  setting  the  shutter  and 
changing  the  plates  we  must  resort  to  some  other  source  of 
power  than  the  observer's  arm.  Air-driven  turbines  or 
propellers  have  been  used  on  aerial  cameras,  as  well  as  clock- 
work, and  also  electric  power,  the  latter  derived  either  irom 
a  generator  or  from  storage  batteries.  The  relative  merits 
of  these  sources  of  power  form  the  subject  of  a  separate 
chapter.  Mention  only  is  here  made  of  the  form  of  drive 
actually  employed  in  connection  with  the  various  cameras. 

The  term  semi-automatic  camera  is  best  used  to  designate 
that  type  in  which  the  observer  (or  pilot)  has  merely  to 
release  the  shutter,  after  which  the  mechanism  performs  all 
the  operations  necessary  to  prepare  for  the  next  exposure. 
There  has  been  some  difference  of  opinion  as  to  whether  it 
is  ever  advisable  to  go  further  than  this  with  plate  cameras. 
The  English  Service  holds  that  completely  automatic  expos- 
ing, in  addition  to  plate  changing,  is  apt  to  encourage  the 
making  of  many  more  pictures  than  necessary,  involving 
carrying  an  excessive  weight  of  plates.  The  French  Service 
has  rather  generally  favored  entirely  automatic  cameras  in 

116 


SEMI-AUTOMATIC   CAMERAS     117 

theory,  although  during  the  war  practically  all  the  work 
of  the  French  army  was  done  by  the  hand-operated  cameras 
already  described. 

The  English  L  Type  Camera. — The  L,  a  modification  of 
the  earlier  C  and  E  models,  differs  from  its  predecessors 
chiefly  in  the  addition  of  a  mechanism  which  when  connected 


FIG.  50. — American  model,  English  "L"  type  semi-automatic  camera. 

with  a  suitable  source  of  power  can  be  used  whenever  desired 
for  changing  the  plates  and  setting  the  shutter.  As  in  the 
C  and  E  types,  all  unexposed  plates  are  carried  in  a  magazine 
above  the  camera,  while  the  exposed  plates  are  shifted  in  a 
horizontal  direction  to  one  side  and  fall  thence  to  a  re- 
ceiving magazine. 

Fig.  50  shows  the  American  model,  which  is  a  copy, 
with  modifications,  of  the  original  English  design.    Its  weight 


118        AIRPLANE  PHOTOGRAPHY 

with  one  loaded  magazine  is  about  35  pounds.  Its  manner 
of  functioning  may  be  studied  from  the  picture  of  the 
mechanism  (Fig.  51).  The  part  of  the  mechanism  to  the 
left  is  inoperative  during  hand  operation,  and  the  large 
toothed  wheel  is  locked  by  the  removable  pin  shown  hanging 
on  its  chain  in  Fig.  50.  To  change  a  plate  and  set  the  shutter 


i 


FIG.  51. — Mechanism  of  "L"  camera. 


the  projecting  lever  (Fig.  50)  is  thrown  over  and  back. 
This  causes  a  sliding  tray,  in  which  the  exposed  plate  rests, 
to  travel  to  the  right,  over  the  receiving  magazine,  where 
the  plate  is  dropped.  After  this  the  tray  returns  to  the  left 
exposing  position.  Simultaneously  the  shutter  is  wound  up. 
Exposure  is  made  either  by  pressing  down  upon  the  plunger, 
or  better,  by  using  a  Bowden  wire.  Provision  for  both 
methods  of  exposing,  one  for  the  pilot  and  one  for  the  obser- 


SEMI-AUTOMATIC  CAMERAS     119 

ver,  is  shown  in  Fig.  81.  The  shutter  is  th  variable- 
aperture  type  already  described,  provided  in  addition  with 
a  tension  adjustment  on  the  back  of  the  camera.  A  flap 
behind  the  lens  does  the  capping  during  the  setting  operation. 

For  power  operation  the  camera  is  connected  through  a 
flexible  shaft  with  a  wind  driven  propeller  (Figs.  50,  83  and 
84).  The  locking  pin  is  now  moved  over  from  the  toothed 
wheel  to  the  lever  arm,  so  that  the  rotation  of  the  worm 
driving  the  large  toothed  wheel  forces  the  lever  through  its 
plate  changing  motion.  To  prevent  repetition,  a  part  of  the 
periphery  of  the  toothed  wheel  is  cut  out,  so  that  it  stops  when 
its  cycle  is  run.  When  the  Bowden  wire  actuates  the  shutter 
release  it  forces  the  toothed  wheel  around  into  engagement 
(aided  by  one  spring  tooth)  and  so  starts  the  cycle  once  more. 

When  connected  with  the  air  propeller  the  worm  is  ro- 
tated continuously.  Other  sources  of  power — an  electric 
motor,  for  instance — can  be  attached  through  the  same  kind 
of  flexible  shaft.  If  an  electric  motor  is  employed  it  may  be 
run  continuously  or  it  may  be  operated  with  an  insulated 
sector  introduced  into  the  large  toothed  wheel  so  that  the 
electric  circuit  is  broken  and  the  motor  stops  until  the  wheel 
is  once  more  forced  around  by  the  exposing  lever. 

Faults  of  the  L  camera.  The  L  camera  was  the  mainstay 
of  the  English  Air  Service.  In  fact  for  the  last  two  years  of 
the  war  it  was  practically  the  only  camera  the  English  used, 
and  they  thought  highly  of  it.  It  is,  of  course,  subject  to  the 
limitation  of  small  plate  size  and  short  focus  lens.  It  is  in 
many  ways  an  inconvenient  camera  to  handle.  For  instance, 
the  upper  magazine  cannot  be  closed  or  removed  until  all 
the  plates  are  passed  through.  Its  dependence  upon  gravity 
for  the  plate  changing  operation  is  a  fundamental  weakness, 
responsible  for  its  frequent  tendency  to  jam  in  the  air. 
Experience  made  the  English  observers  very  expert  in 


120        AIRPLANE  PHOTOGRAPHY 

relieving  these  jams.  Sometimes  they  would  turn  the 
propeller  backward  (mounting  it  in  an  accessible  position 
to  provide  for  this  contingency),  sometimes  they  would 
shake  or  thump  the  camera.  But  while  these  makeshifts 
would  serve  to  secure  pictures — the  chief  object,  of  course,  of 
the  photographic  service — they  can  scarcely  be  said  to  render 
the  camera  satisfactory. 

Moreover,  the  propeller  drive  has  not  been  universally 
approved,  as  it  furnishes  an  additional  mechanism  to  make 
trouble.  Since  it  is  not  feasible  to  change  from  power  to 
hand  operation  while  in  the  air,  the  camera  is  put  out  of 
commission  whenever  the  propeller  or  shaft  is  disabled. 
Bowden-wire  controls  for  both  plate  changing  lever  and 
shutter  release  were  common  in  the  British  service,  which 
considered  the  extra  operation  or  the  extra  muscular  exer- 
tion unimporant  when  compared  with  the  greater  assurance 
of  reliable  action. 

The  English  LB  and  BM  Cameras. — During  the  closing 
months  of  the  war  an  improved  L  type  camera  was  con- 
structed, the  LB.  This  differs  from  the  L  in  a  number  of 
detail  changes,  dictated  by  experience.  The  shutter  is  now 
made  removable  and  self -capping.  Pivoted  lugs  are  pro- 
vided to  hold  the  exposed  plate  horizontal  until  the  very 
instant  it  drops,  in  an  effort  to  prevent  jams  caused  by 
the  plates  piling  up  at  an  angle  in  the  receiving  magazine. 
The  chief  addition,  however,  is  the  provision  of  several 
interchangeable  cones  and  cylinders,  for  carrying  lenses  of 
focal  lengths  from  4  to  20  inches.  Fig.  95  shows  the  LB  with 
20  inch  lens  cylinder  mounted  on  a  bell  crank  support  in 
the  camera  bay  of  an  English  plane. 

The  BM  camera  is  but  a  larger  edition  of  the  LB,  for 
18X24  centimeter  plates.  It  also  carries  several  inter- 
changeable lens  cones. 


SEMI-AUTOMATIC   CAMERAS 

The  American  model  deRam  camera. — The  rotary  chang- 
ing box  devised  by  Lieutenant  deRam  of  the  French  army 
and  incorporated  in  his  entirely  automatic  plate  camera, 
has  been  adapted  by  the  American  Air1  Service  to  a  very 
successful  semi-automatic  camera.  Fig.  52  shows  the  prin- 
ciple of  this  changing  box.  The  pile  of  fifty  plates,  each  in 
its  sheath,  is  carried  in  a  rectangular  box  open  at  top  and 
bottom.  The  lower  plate  next  the  focal-plane  shutter  is 
first  exposed;  the  pile  then  rotates  about  a  horizontal  axis 
through  a  complete  turn.  When  the  exposed  plate  arrives 
in  a  vertical  position  it  is  allowed  to  drop  off,  by  the  opening 
of  cam  actuated  fingers,  and  lodges  against  the  side  of  the 
enclosing  camera  box  proper.  Still  further  along  in  the  cycle 
the  plate  is  thrown  off  from  its  lodging  place  into  a  "scoop" 
on  the  top  of  the  rotating  container  and  laid  on  the  top  of 
the  plate  pile.  Meanwhile  the  curtain  of  the  focal-plane 
shutter  winds  up,  at  the  same  time  that  it  is  depressed  out  of 
the  way  of  the  revolving  plate  container.  Although  the 
plate  changing  operation  depends  on  gravity,  it  nevertheless 
functions  satisfactorily  up  to  30  degrees  from  the  vertical. 

The  shutter  in  this  model  is  the  variable-aperture  fixed- 
tension  type,  adjusted  by  pivoted  idlers  (Fig.  28).  In 
the  exposing  position  it  runs  within  three  millimeters  of  the 
plate  surface,  and  is  therefore  of  high  efficiency  for  all  open- 
ings. Capping  during  the  operation  of  setting  is  performed 
by  flaps  at  the  bottom  of  the  camera  body.  Interchange- 
able cones  are  supplied  for  lenses  of  various  focal  lengths. 

For  hand  operation  the  changing  box  is  turned  over  by 
means  of  a  handle,  which  rotates  four  times  for  the  complete 
cycle  (Fig.  90) .  For  semi-automatic  operation  an  additional 
mechanism  is  provided  on  the  side  of  the  rectangular 
camera  body,  copied  with  some  necessary  modifications 
after  the  L  camera  power  drive.  From  the  observer's  stand- 


AIRPLANE  PHOTOGRAPHY 


BROCK 


jyft^i^ 

i 
=.=^=-•^=1 

-_—  _r-^_-^-j 

FOLMER 


\ J 


v 


o 


FIG.  52. — Diagram  of  automatic  plate  camera  movements. 


SEMI-AUTOMATIC  CAMERAS     123 

point  the  operation  of  the  whole  camera  is  the  same  as  in  the 
L  camera,  with  the  important  exception  that  power  operation 
in  no  way  interferes  with  hand  operation.  Indeed,  the  hand 
can  help  out  if  the  power  flags  or  fails. 

This  camera  is  most  satisfactorily  driven  by  a  12  volt 
1/10  HP  electric  motor  working  through  a  flexible  shaft 
attached  to  a  swivel  connection  at  the  front  of  the  semi- 
automatic drive  box.  A  change  once  every  four  to  five 
seconds  is  possible,  but  greater  speed  is  apt  to  throw  the 
changing  plate  too  violently  for  safety. 

The  chief  practical  objection  to  this  camera  is  its  bulk. 
Its  great  height  makes  it  impractical  for  many  planes.  Its 
weight  of  nearly  a  hundred  pounds  is  a  formidable  load  for 
a  plane  to  carry,  but  this  is  no  more  and  probably  less  than 
that  of  any  other  camera  when  taken  up  with  the  same  num- 
ber of  plates  in  magazines.  The  price  paid  for  economizing  in 
magazine  weight  is  that  the  whole  camera  body,  excluding 
the  lens  cone,  must  be  carried  to  and  from  the  plane  for  "both 
loading  and  unloading. 


CHAPTER  X 
AUTOMATIC  AERIAL  PLATE  CAMERAS 

General  Characteristics. — The  ideal  in  the  automatic- 
plate  camera  is  to  provide  a  mechanism  which  will  not  only 
change  the  plates  and  set  the  shutter,  as  does  the  semi- 
automatic, but  make  the  exposures  as  well,  at  regular  inter- 
vals under  the  control  of  the  operator.  Such  a  wholly  auto- 
matic camera  w^ould  leave  the  observer  entirely  free  for 
other  activities  than  photography  and  it  is  to  meet  this 
tactically  desirable  aim  that  the  war-time  striving  for  auto- 
matic cameras  was  due. 

It  is  obvious  that  the  one  essential  difference  between  the 
automatic  and  semi-automatic  types  lies  in  the  self-contained 
exposing  mechanism  with  its  device  for  the  timing  of  the 
exposures.  There  is  no  difficulty  in  arranging  for  the  driving 
power  to  trip  the  shutter,  but  it  is  no  easy  matter  to  design 
apparatus  which  will  space  the  exposures  equally,  and  at 
the  same  time  permit  of  a  variation  of  the  interval.  It  is 
indeed  the  crux  of  the  problem  of  automatic  camera  design 
to  provide  for  the  easy  and  certain  variation  of  the  interval 
from  the  two  or  three  seconds  demanded  for  low  stereo- 
scopic views  to  the  minute  or  more  that  high  altitude  wide 
angle  mapping  may  permit.  This  problem  is  one  intimately 
bound  up  with  the  question  of  means  of  power  drive  and  its 
regulation,  and  will  be  treated  in  part  in  that  connection. 
It  is  to  be  noted,  however,  that  there  are  in  general  two  modes 
of  exposure  interval  regulation.  One  is  by  variation  in  the 
speed  at  which  the  whole  camera  mechanism  is  driven.  The 
other  is  by  the  mere  addition  to  a  semi-automatic  camera  of 
a  time  controlled  release  which  affects  in  no  way  the  speed 

124 


AUTOMATIC  PLATE   CAMERAS    125 

of  the  plate  changing  operation.  In  many  respects  the  latter 
is  the  best  way  to  make  an  automatic  camera. 

While  the  advantages  of  automatic  cameras  are  great  it 
must  not  be  overlooked  that  a  camera  which  can  only  be 
operated  automatically  is  of  limited  usefulness.  It  is  not 
suited  for  "spotting"  at  any  definite  instant,  as,  for  illustra- 
tion, at  the  moment  of  explosion  of  a  bomb.  It  should,  there- 
fore, be  the  aim  of  the  automatic  camera  designer  to  so 
build  the  apparatus  that  it  can,  at  will,  be  used  semi-auto- 
matically.  In  addition,  to  meet  the  contingency  of  any 
break-down  in  the  source  of  power,  the  camera  should  be 
capable  of  hand  operation,  as  in  the  case  .of  the  American 
semi-automatic  deRam.  In  short,  the  automatic  camera 
should  not  be  a  separate  and  different  type;  it  should  merely 
have  an  additional  method  of  operation. 

Certain  desirable  mechanical  features  of  all  aerial  cameras 
have  already  been  enumerated.  Some  of  these  may  be 
repeated  here  with  the  addition  of  others  peculiar  to  auto- 
matic cameras.  As  a  general  caution,  mechanical  motions 
depending  on  gravity  or  on  springs  should  be  avoided. 
Movements  adversely  affected  by  low  temperatures  (20  to 
30  degrees  below  zero,  Centigrade),  are  unsuitable.  All 
adjustments  called  for  in  the  air  must  be  operable  by  dis- 
tance controls  whoge  parts  are  large,  rugged,  and  not  depend- 
ent on  sound  or  delicate  touch  for  their  correct  setting. 
The  center  of  gravity  of  the  camera  should  not  change 
during  operation  (important  in  connection  \\ith  the  problem 
of  suspension).  The  camera  should  work  in  the  oblique  as 
well  as  in  the  vertical  position.  The  power  required  for 
operation  must  not  exceed  that  available  on  the  plane. 
Electrical  apparatus,  for  instance,  should  not  demand  more 
than  100  watts. 

Any  devices  which  diminish  the  weight  of  the  camera  are 


126        AIRPLANE  PHOTOGRAPHY 

particularly  desirable  in  automatic  plate  cameras,  because  of 
the  large  number  of  exposures  which  such  cameras  encourage. 
For  instance,  if  the  plates  could  be  handled  without  placing 
them  in  metal  sheaths  we  should  gain  a  substantial  reduction 
in  weight  (the  sheaths  weigh  nearly  as  much  as  the  plates) 
as  well  as  in  the  time  necessary  for  handling. 

The  Brock  Automatic  Plate  Camera. — This  camera  is 
somewhat  similar  to  the  same  designer's  film  camera,  both 
in  shape,  in  size,  and  in  its  employment  of  a  heavy  spring 
motor  for  the  driving  power.  It  uses  4X5  inch  plates,  and 
carries  a  10  to  12  inch  lens. 

The  plate-changing  operation  is  unique.  As  shown 
diagrammatically  in  Fig.  52,  the  unexposed  plates  are  carried 
in  a  magazine  on  top  of  the  camera,  the  exposed  ones  in  a 
magazine  inserted  in  the  body  of  the  camera,  directly  below 
the  unexposed  magazine.  The  bottom  plate  of  the  exposed 
pile  drops  into  a  sliding  frame  and  is  carried  along  the  top 
of  the  camera  to  the  exposing  position.  After  exposure,  the 
plate  is  carried  back  and  drops  into  the  receiving  magazine. 
In  order  for  the  plate  to  fall  only  the  proper  distance  at  each 
stage  of  the  cycle,  special  plate  sheaths  are  necessary.  These 
are  cut  away  to  form  edge  patterns  which  clear  or  engage 
control  fingers  so  as  to  ride  or  fall  through  the  sliding  frame 
as  required. 

The  camera  is  entirely  automatic  in  operation.  Regula- 
tion of  the  exposure  interval  is  by  a  special  form  of  variable 
length  escapement  controlled  through  a  Bowden  wire,  in  a 
manner  parallel  to  that  in  the  Brock  film  camera,  described 
elsewhere.  These  plate  cameras  were  never  produced 
in  quantity. 

Folmer  13X18  Centimeter  Automatic  Camera. — This 
camera,  also  never  manufactured  in  quantity,  is  shown  in 
Fig.  53,  and  a  sketch  of  its  manner  of  operation  is  included  in 


AUTOMATIC  PLATE  CAMERAS 


the  ensemble  of  automatic  camera  diagrams  (Fig.  52).  Its 
most  distinctive  feature  is  perhaps  the  use  of  a  two  compart- 
ment magazine.  This  is  similar  in  form  to  the  one  already 
described  in  connection  with  the  hand-held  cameras,  but 
larger,  to  hold  eighteen  13X18  centimeter  plates.  The 
unexposed  plates  are  placed  in  one  compartment,  and  after 
exposure  are  shifted  to  the  other.  The  transfer  is  effected 


FIG.  53. — Folmer  13x18  centimeter  automatic  and  semi-automatic  plate  camera. 

by  the  motion  of  a  rack,  which  is  part  of  the  magazine  and 
which  is  driven  by  a  toothed  pinion,  also  part  of  the  magazine, 
which  in  turn  engages  in  a  toothed  wheel  projecting  upward 
from  the  camera  body.  This  toothed  wheel  is  turned  first 
in  one  direction  and  then  in  the  other  by  an  arrangement  of 
gears  and  levers  driven  by  the  source  of  power,  which  as 
shown  in  Fig.  53  is  a  wind  turbine  connected  through  a 
flexible  shaft.  Operation  is  either  automatic  or  semi- 
automatic as  desired,  and  the  camera  can  be  put  through 
its  cycle  by  hand  if  necessary. 


128        AIRPLANE  PHOTOGRAPHY 


FIG.  54.— French  model  deRam  automatic  plate  camera. 


AUTOMATIC  PLATE  CAMERAS 

As  with  several  other  designs,  the  completion  of  the  work- 
ing model  of  this  camera  occurred  after  agreements  had 
been  reached  by  the  Allies,  as  to  plate  size,  standard  lens 
cones,  and  other  features,  not  easily  incorporated  in  it,  thus 
making  manufacture  inadvisable.  The  validity  of  the  design 
for  peace-time  work  is,  of  course,  not  affected  by  this  fact. 

The  deRam  Camera. — The  only  completely  automatic 
plate  camera  actually  produced  commercially  before  the 
end  of  hostilities  was  the  French  model  deRam  (Fig.  54). 
Its  plate-changing  action  has  already  been  described  in 
connection  with  the  American  semi-automatic  model  (Figs. 
52,  90  and  91).  It  differs  from  the  American  model  in  the 
shutter,  which  is  of  the  self-capping  variety,  carried  on  a 
rising  and  falling  frame;  and  in  the  exposing  mechanism. 
The  latter  embodies  a  clutch  whose  point  of  attachment  to 
a  uniformly  rotating  disc  in  the  camera  is  governed  through  a 
Bowden  wire,  whereby  the  interval  between  the  plate-chang- 
ing operation  and  the  shutter  release  is  varied.  The  inter- 
vals are  indicated  by  figures  on  the  dial  to  which  the  observ- 
er's end  of  the  Bowden  wire  is  attached.  The  source  of 
power  for  the  camera  is  a  constant  speed  propeller.  Complete 
semi-automatic  operation  is  not  possible,  as  an  interval  of  1 
to  2  seconds  elapses  between  the  time  a  single  exposure  is 
called  for  and  its  occurrence.  No  arrangement  is  provided 
for  hand  operation. 

It  will  be  noted  that  while  this  camera  is  a  true  automatic 
apparatus  it  does  not  meet  even  a  majority  of  the  require- 
ments listed  above  as  found  desirable  by  experience.  There 
exists  a  great  opportunity  for  designing  and  developing  an 
entirely  satisfactory  automatic  plate  camera — provided  it  is 
agreed  that  anything  more  than  semi-automatic  operation 
is  ever  advisable  for  plates. 


CHAPTER  XI 
AERIAL  FILM  CAMERAS 

The  weight  of  the  glass  and  the  sheaths  in  the  plate 
camera  forms  its  most  serious  drawback.  This  weight  must 
be  reckoned  at  least  three  quarters,  of  a  pound  for  each 
18  X24  centimeter  plate.  Consequently,  with  the  use  of  these 
large  plates,  and  with  the  demands  for  ever  increasing  num- 
bers of  pictures  to  be  taken  on  long  reconnaissance  flights,  a 
serious  conflict  arises  between  the  weight  of  the  photographic 
equipment  and  the  carrying  capacity  of  the  plane.  Among 
plate  cameras  probably  the  most  economical  in  weight  is 
the  deRam.  It  carries  fifty  18X24  centimeter  plates,  and 
has  a  total  weight  of  approximately  100  pounds.  An  ad- 
vance to  100  or  200  plates — not  feasible  in  the  deRam  con- 
struction— even  if  we  assume  the  lightest  possible  magazines, 
would  bring  the  weight  of  camera  and  plates  to  150  or  200 
pounds,  which  would  be  detrimental  to  the  balance  and 
would  seriously  infringe  on  the  fuel  carrying  capacity  and 
ceiling  of  any  ordinary  reconnaissance  plane. 

Early  and  persistent  attention  was  therefore  paid  to  the 
possibilities  of  celluloid  film  in  rolls,  as  used  so  widely  in 
hand  cameras  and  in  moving  picture  work.  The  two  great 
advantages  of  film  would  be  its  practically  negligible  weight 
(approximately  one-tenth  that  of  plates,  not  including 
sheaths)  and  its  small  bulk,  which  would  permit  the  greatest 
freedom  in  the  development  of  entirely  automatic  cameras 
to  make  exposures  by  the  hundreds  instead  of  by  the  dozens. 
Certain  disadvantages  were  foreseen  at  the  outset :  •  the 
difficulty  of  holding  the  film  flat  and  immune  from  vibration 
in  the  larger  sizes;  the  difficulty  of  quickly  developing  and 

130 


AERIAL  FILM  CAMERAS  131 

drying  large  rolls;  the  question  whether  as  good  speed  or 
color  sensitiveness  could  be  obtained  in  sensitive  emulsions 
when  flowed  on  a  celluloid  base  as  on  glass.  Early  trials 
revealed  a  further  problem  to  solve:  how  to  eliminate  the 
discharge  of  static  electricity  occurring  at  high  altitudes, 
especially  when  the  weather  is  cold. 

As  far  as  camera  construction  is  concerned  the  chief 
problems  are  to  hold  the  film  flat,  and  to  eliminate  static. 

Methods  of  Holding  Film  Flat. — Several  means  have  been 
proposed  and  used  for  holding  the  film  flat.  Disregarding 
mere  pressure  guides  at  the  side,  which  are  suitable  only  for 
small  area  films  (up  to  4X5  inch),  the  successful  means 
have  taken  three  forms:  pressure  of  a  glass  plate,  pressure 
of  the  shutter  curtain,  and  suction.  A  glass  pressure  plate 
can  be  used  in  either  of  two  ways;  the  film  may  be  in  con- 
tinuous contact  with  it  or  may  be  pressed  against  its  surface 
only  at  the  moment  of  exposure.  The  advantage  of  this 
first  method  lies  solely  in  its  mechanical  simplicity;  its  dis- 
advantage in  the  likelihood  of  scratches  or  pressure  markings 
on  the  film.  Where  a  glass  plate  is  used  there  is  always  the 
chance  of  a  dust  or  dirt  film  accumulating,  or  of  the  conden- 
sation of  moisture,  to  impair  the  quality  of  the  negative. 
There  is,  moreover,  an  inevitable  loss  of  light  (about  10%), 
together  with  some  slight  distortion,  due  to  the  bending  of 
the  marginal  oblique  rays  through  the  thickness  of  the  glass. 
In  cases  where  a  filter  would  normally  be  employed,  the  loss 
of  light  is  minimized  by  using  yellow  glass  for  the  plate,  so 
that  it  serves  for  filter  and  film  holder  as  well. 

Pressure  of  the  shutter  curtain  is  utilized  in  the  Duch- 
atellier  film  camera  by  furnishing  the  edges  of  the  curtain 
aperture  with  heavy  velvet  strips,  whose  light  and  gentle 
pressure  during  the  passage  of  the  shutter  holds  the  film 
against  a  metal  back.  In  many  ways  this  is  the  simplest 


132        AIRPLANE  PHOTOGRAPHY 

film-holding  device;  it  occasions  no  loss  of  light,  and  needs 
no  mechanical  movements  or  external  accessories,  such  as  are 
called  for  in  the  suction  devices  next  described.  There  is 
always  danger  of  markings  on  the  film,  if  the  velvet  is  not  of 
the  right  thickness  and  softness,  and  the  operation  and  speed 
control  of  the  shutter  are  necessarily  complicated  by  the 
additional  frictional  load. 

Suction  of  the  film  against  a  perforated  back  plate  has 
been  found  a  very  successful  means  of  securing  flatness. 
Suction  at  the  moment  of  exposure  may  be  produced  by  the 
action  of  a  bellows,  which  has  been  compressed  beforehand 
by  the  camera-driving  mechanism.  Continuous  suction  can 
be  produced  either  by  a  continuously  driven  pump,  or  by  a 
Venturi  tube  placed  outside  the  fuselage.  The  Venturi  tube 
(Fig.  55)  consists  of  a  pipe  built  up  of  two  cones,  placed 
vertex  to  vertex,  to  form  a  constriction.  When  air  is  forced 
through  this  at  high  velocity  suction  is  produced  in  a  small 
diameter  tube  taken  off  at  the  constriction.  A  suction  of  two 
centimeters  of  mercury,  acting  through  holes  about  one  centi- 
meter equidistant  from  each  other  in  the  back  plate,  has  been 
found  adequate  to  hold  flat  a  film  18X24  centimeters. 

One  merit  of  suction  applied  only  at  the  moment  of 
exposure  is  that  the  film-driving  mechanism  does  not  have 
to  work  against  the  drag  of  the  suction.  Continuous  suction, 
on  the  other  hand,  gives  a  longer  opportunity  for  flattening 
out  kinks  in  the  celluloid,  and  easily  permits  movement  of 
the  film  during  the  exposure,  either  for  the  purpose  of  per- 
mitting a  longer  exposure  or  for  the  purpose  of  preventing 
distortion  due  to  the  focal -plane  shutter.  A  disadvantage 
of  continuous  suction  is  the  production  of  minute  scratches 
on  the  celluloid  surface  as  it  drags  over  the  suction  plate. 
These  are  ordinarily  too  small  to  cause  trouble,  but  may  show 
up  when  printing  is  done  in  an  enlarging  camera. 


AERIAL  FILM  CAMERAS 


133 


Static  discharges  are  produced  by  the  friction  of  the 
celluloid  against  the  pressure  back  or  other  surfaces  with 
which  it  comes  into  contact.  They  show  in  the  developed 
film  as  branching  tree-like  streaks  (Fig.  56)  and  in  cold  dry 
weather  may  be  numerous  enough  to  ruin  a  picture.  The  dis- 


J 

JP 


FIG.  55. — Venturi  tube  on  side  of  plane. 

charges  are  noticeably  less  frequent  with  film  coated  on  the  back 
with  gelatine  ("N.C."),  but  the  extra  gelatine  surface  is  ex- 
tremely undesirable.  When  handled  by  developing  machines, 
as  large  rolls  must  be,  this  back  gelatine  surface  becomes 
scratched  and  bruised  in  a  serious  manner.  Plain  unbacked 
film  is  much  to  be  preferred  if  the  static  can  be  obviated. 
To  avoid  static,  it  is  necessary  to  provide  for  the  imme- 
diate dissipation  of  all  acquired  electrical  charges.  Experi- 


134        AIRPLANE  PHOTOGRAPHY 

ments  made  by  the  United  States  Air  Service  have  shown 
that  nothing  is  so  good  as  rather  rough  cloth,  thoroughly 
impregnated  with  graphite,  held  in  close  contact  with  the 
celluloid  during  as  great  a  portion  of  its  travel  as  possible. 
In  the  United  States  Air  Service  film  camera  which  uses 


FIG.  56. — Print  from  film  camera  negative,  showing  static  discharge,  and  (upper  left-hand  corner) 
record  of  altitude  and  compass  direction  made  by  Williamson  film  camera_auxiliary  lens  (Fig.  58). 

suction  through  a  perforated  back  plate,  the  plate  has  been 
covered  with  thin  graphited  cloth,  and  similar  cloth  sheets 
are  pressed  against  the  film  rolls  by  sheets  of  spring  metal 
(Fig.  65).  In  cameras  with  this  equipment  no  trouble  has 
been  experienced  with  static. 

Representative    Film    Cameras. — The    English    F    type 
(Williamson).     This  is  one  of  the  earliest  cameras  designed 


AERIAL  FILM  CAM  ERAS'  135 

for  film,  as  is  indicated  by  the  nature  of  the  power  drive, 
which  presupposes  that  the  camera  is  to  be  carried  on  the 
outside  of  the  fuselage.  Its  essential  features  are  shown  in 
Figs.  57  and  58.  It  consists  of  a  rectangular  box  with  a  cone 
at  the  front  on  which  is  mounted  a  propeller,  intended  to  be 
rotated  by  the  wind  made  by  the  motion  of  the  plane.  This 
drives,  through  a  governor  controlled  friction  clutch,  a  train 
of  gears  which  draws  the  (5X4  inch)  film  across  the  focal 
plane,  sets  and  exposes  the  shutter  at  regular  intervals. 


FIG.  57. — English  type  "F"  (Williamson)  automatic  film  camera. 

Above  the  camera,  supported  on  a  tripod,  are  a  compass 
and  altimeter,  both  recording  on  a  single  dial,  illuminated 
from  below  by  the  light  reflected  from  a  circular  white  disc 
painted  on  top  of  the  camera.  An  image  of  the  dial  is  thrown 
on  a  corner  of  the  film  by  a  lens,  whose  shutter  is  actuated 
in  synchronism  with  the  main  focal-plane  shutter.  No 
special  means  are  provided  for  holding  the  film  flat.  Special 
film  with  perforated  edges  is  used. 

The  camera  was  designed  for  mapping  work  on  the 
Mesopotamian  and  other  fronts  where  no  maps  at  all  existed. 


136        AIRPLANE  PHOTOGRAPHY 

The  Duchatellier  camera  is  essentially  a  film  magazine  to 
fit  on  the  standard  French  deMaria  camera  .bodies,  of  the 
18X24  centimeter  size.  In  its  simplest  form  it  embodies  a 
shutter  (the  regular  focal-plane  shutter  of  the  camera  being 
removed)  and  a  film-moving  mechanism,  both  actuated  by 
a  single  motion  of  the  hand.  Automatic  and  semi-automatic 
operation  are  accomplished  by  an  auxiliary  mechanism  to 


FIG.  58. — Interior  of  type  "F  "  camera,  showing  lens  for  photographing  compass  and  altitude  readings. 

which  Bowden  wires  from  the  hand  lever  are  attached.  The 
motive  power  is  an  air  propeller.  Variation  of  speed  is  ob- 
tained by  changing  the  point  of  contact  of  a  roller  on  a  friction 
disc,  the  disc  being  directly  connected  to  the  propeller  shaft, 
the  roller  to  the  camera  drive  shaft. 

The  most  distinctive  features  of  the  Duchatellier  camera 
is  its  use  of  the  focal-plane  shutter  to  hold  the  film  flat  during 
the  exposure.  As  already  explained,  this  is  accomplished  by 
pressure,  velvet  strips  on  the  shutter  edges  keeping  the  film 
close  against  the  back  plate.  The  return  of  the  shutter 


AERIAL  FILM   CAMERAS 


137 


curtain  to  the  "set"  position  is  accomplished  by  locking  it 
to  the  film  by  perforating  points,  so  that  it  is  pulled  across 
as  the  film  is  wound.  This  introduces  between  each  pair  of 
pictures  a  strip  of  tremendous  over  exposure,  as  wide  as  the 


FIG.  59. — G.  E.  M.  automatic  film  camera. 

curtain  opening.  A  fixed-aperture  variable-tension  shutter 
is  used.  The  magazine  carries  a  roll  of  film  long  enough  for 
200  exposures,  feeding  the  long  way  of  the  picture.  When 
film  needs  to  be  changed  in  the  air,  this  is  done  by  changing 
the  entire  magazine,  including  its  shutter. 


138 


AIRPLANE  PHOTOGRAPHY 


The  G.  E.  M.  camera  (Fig.  59)  is  a  very  light  self-contained 
clockwork-drive  camera  taking  36  pictures  six  inches  square. 
The  film  is  unrolled  from  a  small-diameter  feeding  roller  on 
to  a  large-diameter  receiving  roller  to  which  the  driving 
mechanism  is  attached.  By  this  means  approximately  equal 
spacing  of  pictures  on  the  film  is  assured.  The  film  is  held 
flat  by  continuous  contact  with  a  glass  plate,  which  is  made  of 
yellow  glass,  so  that  it  serves  at  the  same  time  as  a  color  filter. 


"  '• 


Fid.  60. — Brock  automatic  film  camera. 

The  lens — of  8  to  12  inch  focus — is  equipped  with  a  single 
speed  between-the-lens  shutter.  The  operation  of  the 
camera  is  entirely  automatic.  The  interval  between  pictures 
is  controlled  by  varying  the  clockwork  speed,  through  a  lever 
on  the  outside  of  the  camera  box.  Protection  of  the  camera 
from  vibration  is  sought  by  supporting  it  on  four  spring 
cushions  mounted  on  a  solid  frame,  to  which  the  camera  is 
held  by  spiral  springs  attached  to  its  sides. 

The  Brock  Film  camera  (Fig.  60)  is  an  entirely  automatic, 


AERIAL  FILM  CAMERAS  139 

very  compact  self-contained  camera,  taking  one  hundred 
4X5  inch  pictures.  The  motive  power  is  clockwork,  regu- 
lated in  speed  by  an  escapement  controlled  by  a  flexible  shaft 
carried  to  a  dial  which  may  be  fastened  to  the  instrument  board 
or  to  some  other  convenient  part  of  the  plane.  The  lens  is 
6,  12,  or  18  inch  focus.  The  shutter  is  of  the  fixed-aperture 
variable-tension  type,  of  long  travel,  and  with  a  flap  behind 
the  lens  for  covering  during  the  setting  period.  None  of  the 
special  means  above  described  for  holding  the  film  flat  are 
provided.  A  metal  plate  resting  on  the  back,  and  a  flat 
metal  frame  in  front  with  a  4  X5  inch  aperture,  are  considered 
sufficient  check  on  the  excursions  of  the  small-sized  film. 
A  ball  bearing  double  pivoted  frame  serves  to  support  the 
camera  in  a  pendulous  manner,  permitting  it  to  assume  a 
vertical  position  after  tilting.  Damping  of  oscillations  and 
vibration  is  arranged  for  by  two  pneumatic  dash  pots. 

The  German  film  mapping  camera,  shown  in  Fig.  61,  is 
distinguished  by  a  number  of  special  features.  The  size  of 
the  pictures,  6X24  centimeters,  is  unusual.  It  has  its  ad- 
vantages, however.  Since  the  short  dimension  is  in  the  line 
of  flight,  the  maximum  width  of  field  covered  by  the  lens  is 
utilized  (Fig.  17).  This  of  course  necessitates  a  larger  num- 
ber of  exposures  to  complete  a  strip,  which  is  perhaps  an 
added  advantage,  since  the  narrower  the  individual  pictures 
the  better  the  junctions  will  be,  especially  if  large  overlaps 
are  made.  This  proved  to  be  the  case  with  captured  German 
mosaics.  Difficulty  is  experienced  in  making  overlaps  on  a 
turn  (Fig.  62),  but  this  is  not  a  vital  objection.  The  shutter 
has  a  fixed  aperture,  narrower  at  the  center  than  at  the  ends, 
to  compensate  for  the  falling  off  in  illumination  away  from 
the  center  of  the  lens.  No  safety  flap  is  needed  because  the 
curtain  moves  in  opposite  directions  on  successive  exposures, 
thereby  also  compensating  for  shutter  distortion,  as  has 


140        AIRPLANE  PHOTOGRAPHY 

already  been  discussed.    Shutter  speed  is  controlled  by  vary- 
ing the  tension  of  the  actuating  spring. 


FIG.  61. — German  automatic  film  camera. 


The  camera  is  driven  by  an  electric  motor,  connected  to  a 
set  of  gears,  whose  shifting  provides  for  speed  variation. 
The  film  is  moved  by  rubber  rollers  which  are  cut  away  for 


AERIAL  FILM  CAMERAS 


141 


652.  —  Method  of  joining  and  printing  film  from  German  camera. 


part  of  the  circumference,  allowing  the  film  to  stand  still 
until  they  bite  again.  A  yellow  glass  pressure  plate  holds  the 
film  during  the  exposure  and  serves  as  color  filter  also 


142        AIRPLANE  PHOTOGRAPHY 

(Fig.  63).  An  electric  heater  is  provided  near  the  shutter,  as 
in  all  the  later  German  cameras. 

United  States  Air  Service  automatic  film  camera — Type  K 
(Figs.  64,  65,  92,  93,  98,  99).  This  is  an  entirely  automatic 
camera,  manufactured  by  the  Folmer  and  Schwing  Division 
of  the  Eastman  Kodak  Co.,  taking  100  pictures  of  18X24 
centimeter  size  at  one  loading.  As  with  all  the  American 
cameras  of  this  size,  it  uses  the  standard  lens  cones  of  any 
desired  focal  length.  The  camera  proper  consists  of  a  com- 
pact chamber  in  which  the  film  rollers  are  carried  at  each  end 
forward  of  the  focal  plane,  the  shutter  lying  between.  In 
consequence  of  this  arrangment  the  vertical  depth  of  the 
camera  is  the  absolute  minimum — short  of  decreasing  the 
length  of  the  optical  path  by  mirror  arrangements — making 
it  possible  to  suspend  the  camera  diagonally  in  the  American 
and  British  planes,  for  taking  oblique  pictures. 

Flatness  of  the  film  is  secured  by  a  suction  plate  covered 
with  graphited  cloth  and  connected  with  a  Venturi  tube. 
The  top  cover  is  removed  for  re-loading.  The  shutters  on 
the  first  cameras  of  this  type  are  of  the  variable-tension 
fixed-aperture  design,  though  later  ones  have  the  variable- 
aperture  curtain  controlled  by  an  idler,  as  used  in  the  Ameri- 
can deRam.  An  auxiliary  curtain  shutter  serves  to  cap  the 
true  shutter  during  setting. 

The  operation  of  the  film  driving  mechanism  is  compara- 
tively simple.  It  consists  of  a  train  of  gears,  driven  by  a 
flexible  revolving  shaft  attached  to  some  separate  source  of 
power  capable  of  speed  variation.  The  action  of  the  gears 
is  to  move  the  film,  set  the  shutter  and  then  expose  it;  in  the 
earlier  cameras  with  the  film  continuously  moving.  In  the 
first  cameras  constructed  the  space  between  the  pictures 
varies  as  the  film  rolls  up,  due  to  the  increasing  diameter  of 
the  roll.  In  later  cameras  the  film  roller  is  disengaged  from 


AERIAL  FILM  CAMERAS 


143 


Q 


FlG.  63. — Film  winding  and  exposing  mechanism  in  German  film  camera. 


144        AIRPLANEPHOTOGRAPHY 

the  gears  just  before  the  shutter  is  tripped,  so  that  the  film 
stands  still  during  the  exposure,  and  is  then  re-engaged  at  a 
new  point  on  a  ratchet  wheel  governed  by  the  diameter  of 
the  receiving  roll,  whereby  the  pictures  are  equally  spaced. 
In  all  the  cameras,  punch  marks  made  at  the  time  of  exposure 
enable  the  limits  of  the  picture  to  be  detected  in  the  dark 
room  by  touch. 

Variable  speed  is  arranged  for  in  any  one  of  several  ways. 
For  peace-time  uses  a  turbine  attached  to  the  side  of  the  plane 
is  simple  and  positive,  and,  provided  it  is  made  of  sufficient 
size — which  is  not  the  case  with  the  one  shown  in  the  Figure 
—will  give  adequate  speed  regulation  upon  varying  the 
aperture  through  which  the  air  enters.  The  Venturi  tube 
may  be  carried  upon  the  same  mount,  or  a  small  rotary  pump 
can  be  attached  on  the  same  shaft.  Where  the  high  wind 
resistance  of  the  turbine  is  an  objection  the  camera  is  driven 
electrically,  by  a  motor  acting  through  the  intermediary  of 
a  variable  speed  control  described  in  the  next  chapter 
(Fig.  68). 

The  camera  weighs  complete  about  forty  pounds,  and 
the  film  rolls  about  four  pounds.  The  latter  can  be  changed 
in  the  air  without  great  difficulty  provided  the  camera  is 
mounted  accessibly  and  so  that  the  top  may  be  opened. 


CHAPTER  XII 
MOTIVE  POWER  FOR  AERIAL  CAMERAS 

As  long  as  circumstances  permit,  hand  operation  still 
remains  the  most  reliable  and  satisfactory  method  of  driving  a 
camera.  It  is  always  available,  can  be  applied  to  just  the 
amount  desired,  and  at  the  time  and  place  needed.  For 
instance, in  a  magazine  of  the  Gaumont  type  (Fig.  40),  what  is 
needed  is  the  periodic  application  of  a  very  considerable 
force  rather  quickly,  and  while  this  can  be  done  quite  simply 
byhand,no  mechanism  has  even  been  attempted  to  go  through 
this  same  operation  automatically.  Instead,  the  fundamental 
design  of  automatic  magazines  has  been  made  along  other 
lines  calculated  to  utilize  smaller  forces  more  steadily  applied. 

It  must  be  granted,  however,  that  for  war  planes,  and 
particularly  for  single  seaters,  cameras  should  be  available 
which  are  capable  of  operating  semi-automatically  or  auto- 
matically. This  necessarily  means  the  employment  of  arti- 
ficial power,  whose  generation,  transmission  to  the  camera 
and  control  as  to  speed  present  a  mechanical  problem  of  no 
small  difficulty. 

Available  Sources  of  Power. — The  sources  from  which 
power  may  be  drawn  on  the  plane  are  four,  although  the 
various  combinations  of  these  present  a  large  number  of 
alternative  approaches  to  the  problem.  These  sources  are: 

1.  The  engine  of  the  plane. 

2.  Wind  motors. 

3.  Spring  motors. 

4.  Electric  motors. 

10  145 


146        AIRPLANE  PHOTOGRAPHY 

These  may  first  be  considered  largely  from  the  descrip- 
tive standpoint,  leaving  questions  of  performance  and  effi- 
ciency for  separate  treatment. 

Power  may  be  derived  directly  from  the  engine  through  a 
flexible  shaft,  similar  to  that  used  for  the  revolution  counter. 
This  source  of  power  is  inherently  the  most  direct  and  effi- 
cient, since  the  engine  is  the  seat  of  all  the  lifting  and  driving 
energy  of  the  plane.  There  is  no  loss  through  transformation 
into  other  forms  of  energy,  such  as  electrical;  or  by  the  use  of 
more  or  less  inefficient  intermediary  apparatus,  such  as  wind 
propellers.  Against  the  direct  drive  of  the  camera  from 
the  engine  may,  however,  be  urged  that  the  usual  distance 
between  engine  and  camera  is  too  great  for  reliable  mechani- 
cal connection,  as  by  flexible  shafting.  Objections  also  arise 
from  the  standpoint  of  speed.  This  cannot  be  controlled  by 
the  camera  operator;  and  varies  over  too  wide  a  range,  as  the 
engine  changes  from  idling  to  full  speed,  to  fit  it  for  automatic 
camera  operation.  The  first  objection  may  be  met  by  that 
combination  of  methods  of  power  drive  which  consists  in 
transmitting  the  power  electrically;  that  is,  by  letting  the 
engine  operate  a  generator  from  which  cables  run  to  a  motor 
close  to  the  camera.  This  method,  of  course,  sacrifices 
efficiency,  and  it  breaks  down  when  the  engine  speed  drops 
below  the  speed  necessary  to  generate  the  requisite  voltage. 
This  defect  may  in  turn  be  met  by  floating  in  storage  batter- 
ies, which  brings  up  the  whole  question  of  electrical  drive, 
to  be  treated  presently.  While  use  of  the  engine  for  direct 
drive  or  for  generating  electric  current  has  not  been  adopted 
in  the  American  service,  it  is  known  that  some  German 
planes  were  supplied  with  electric  current  in  this  way. 

Coming  next  to  the  wind  motors,  these  possess  one  very 
great  merit:  they  utilize  a  motive  power  that  is  always 
present  as  long  as  the  plane  is  in  motion  through  the  air. 


MOTIVE  POWER  147 

On  the  other  hand,  the  process  of  using  the  main  propeller 
of  the  plane  to  pull  another  smaller  propeller  through  the 
air  appears  a  roundabout  way  to  utilize  the  driving  power 
of  the  airplane  engine.  Yet  on  the  whole  it  is  probable  that 
some  form  of  propeller  or  wind  turbine  is  the  simplest  and 
most  convenient  device  we  have  for  the  operation  of  airplane 
auxiliaries.  As  long  as  the  amount  of  power  required  is 
small,  such  inefficiency  as  is  inherent  in  its  use  is  offset  by 
its  convenience  and  reliability.  An  advantage  of  the  pro- 
peller is  that  its  speed  is  almost  directly  proportional  to  that 
of  the  plane  through  the  air,  a  desirable  feature  in  auto- 
matic cameras  provided  the  proportionality  is  under  control. 
Yet  it  is  just  in  this  matter  of  varying  the  speed  at  will  that 
the  propeller  presents  difficulties,  to  be  met  only  by  additional 
mechanisms  for  gearing  down  or  governing.  Propellers 
have  the  practical  disadvantage  that  they  present  an  easily 
bent  or  broken  projection  to  the  body  of  the  plane  (Figs.  83 
and  84).  The  strength  of  small  propellers  for  operating 
auxiliaries  is  never  so  much  in  question  with  reference  to 
their  resistance  to  whirling  and  thrust  of  air  as  it  is  to  their 
ability  to  withstand  the  inevitable  knocks  and  careless  hand- 
ling that  will  fall  to  their  lot.  The  propeller  bracket  is  just 
what  the  pilot  is  looking  for  to  scrape  the  mud  off  his  boots 
before  climbing  in. 

The  wind  turbine  has  the  advantage  over  the  propeller 
that  its  speed  can  be  varied  rather  simply  by  exposing  more 
or  less  of  its  face  to  the  wind.  A  turbine  fitted  with  an 
adjustable  aperture  for  admitting  the  wind  is  shown  in  Fig. 
64,  in  connection  with  the  type  K  automatic  film  camera. 
The  turbine  has  the  advantage  of  being  compact  and  lying 
close  against  the  body  of  the  plane.  In  the  form  figured, 
altogether  too  much  head  resistance  is  offered — just  as  much 
for  low  as  for  high  speeds— but  with  proper  design  this  need 


148        AIRPLANE  PHOTOGRAPHY 

*mjjjjjj*jjjjjm 


MOTIVE  POWER  149 

not  be  the  case.  It  is,  moreover,  quite  too  small  to  give  the 
needed  speed  regulation,  as  it  only  begins  to  operate  near  its 
full  opening. 

Spring  motors  have  the  very  real  advantage  that  by  their 
use  the  camera  can  be  made  entirely  self  contained.  The 
simplest  application  of  the  spring  motor  would  be  to  the  semi- 
automatic camera,  where  no  close  regulation  of  speed  is 
required.  In  such  a  camera  the  operation  of  exposing  the 
shutter  would  release  the  spring,  which  would  then  change 
the  plate  or  film  and  re-set  the  shutter,  repeating  this  opera- 
tion as  long  as  the  spring  retained  sufficient  tension.  Small 
film  hand -cameras  of  this  type,  using  self -setting  between- 
the-lens  shutters,  have  been  designed,  though  not  for  aerial 
work.  The  possibilities  of  using  springs  as  motive  power 
in  semi-automatic  cameras  have  not  apparently  been  seri- 
ously considered. 

When  a  spring  motor  is  used  for  automatic  camera  opera- 
tion it  at  once  becomes  necessary  to  add  to  the  motor  an 
elaborate  clock  mechanism  for  controlling  and  regulating  its 
speed  of  action.  Springs  are  much  better  fitted  for  giving 
power  by  quick  release  of  their  tension  than  by  slow  release, 
and  the  necessary  clock  mechanisms  for  their  regulation 
become  very  heavy,  as  well  as  complicated  and  delicate, 
when  they  are  made  large  enough  to  do  any  real  work.  For 
their  repair  they  require  the  services  of  clock  makers  rather 
than  the  usual  more  available  kind  of  mechanic. 

Coming  next  to  electric  motors,  we  meet  with  a  source  of 
power  of  very  great  flexibility  both  in  its  derivation  and  in 
its  application.  If  a  source  of  electric  current  is  already  pro- 
vided for  heating  and  lighting  as  it  is  on  the  fully  equipped 
military  plane,  and  if  it  has  sufficient  capacity  to  handle  the 
camera,  its  use  is  rather  clearly  indicated,  irrespective  of 
how  efficiently  or  by  what  method  it  is  produced.  Especially 


150        AIRPLANE  PHOTOGRAPHY 

is  this  the  case,  from  the  standpoint  of  economy  and  simplic- 
ity, if  a  propeller-driven  generator  is  the  source  of  current, 
and  the  alternative  power  drive  is  an  additional  propeller 
for  the  camera.  If,  on  the  other  hand,  the  camera  must  have 
its  own  source  of  electric  power,  the  advantages  and  dis- 
advantages must  be  closely  scrutinized.  In  this  case  either 
a  generator  must  be  provided,  or  resort  be  made  to  storage 
batteries,  or  a  combination  of  the  two. 

Ruling  out  a  special  propeller-driven  generator,  we  are 
left  with  either  the  generator  driven  from  the  engine  or  the 
storage  battery.  Inasmuch  as  storage  batteries  are  prac- 
tically indispensable  with  generators,  in  order  to  maintain  the 
voltage  constant  at  all  speeds,  it  is  on  the  whole  advisable 
to  rely  upon  batteries  alone.  An  advantage  of  their  use  is 
that  the  power  plant  is  entirely  within  the  plane:  All  pro- 
jections such  as  propellers  are  avoided.  Another  merit  is  that 
the  power  is  drawn  upon  only  as  needed.  Against  storage 
batteries  is  their  weight,  the  need  for  frequent  charging, 
and  their  loss  of  efficiency  at  low  temperatures — a  loss  so 
serious  with  those  of  the  Edison  form  as  to  preclude  their  use. 

When  once  the  source  of  electrical  energy  is  decided  upon, 
its  method  of  application  needs  to  be  considered.  Here  we 
meet  at  once  the  peculiar  merit  of  electrical  energy,  namely, 
the  ease  and  convenience  with  which  it  may  be  transmitted. 
All  we  need  is  a  pair  of  wires,  led  to  any  part  of  the  plane  by 
any  convenient  route  and  connected  by  simple  binding  posts. 
It  may  with  equal  ease  be  turned  on  or  off  by  merely  making 
or  breaking  a  contact  with  a  switch.  For  operating  semi- 
automatic cameras  this  feature  may  be  utilized  in  the  interest 
of  economy,  if  the  power  is  automatically  turned  off  as  soon 
as  the  plate-changing  operation  is  finished.  Exceptionally 
reliable  make  and  break  contacts  are  necessary  to  insure  the 
success  of  this  latter  scheme. 


MOTIVE  POWER  151 

Two  methods  of  transforming  the  supply  of  electrical 
energy  into  mechanical  motion  are  available.  The  first  is 
by  the  use  of  a  solenoid  and  plunger.  This  is  a  device  prac- 
tically restricted  to  semi-automatic  cameras,  in  which  the 
operation  consists  of  a  straight  to-and-fro  motion,  initiated 
at  the  will  of  the  operator.  It  has  been  used  little  if  at  all. 
The  second  motion  is  the  continuous  rotary  one  secured  by 
the  use  of  an  electric  motor.  This  motion  is  the  most  prac- 
tical one  for  the  continuous  operation  of  any  mechanism,  but 
on  the  other  hand  requires  that  the  imposed  load  be  reason- 
ably uniform  at  all  times  through  the  cycle  of  operations. 
Assuming  that  the  camera  mechanism  is  of  this  character, 
the  motor  may  be  attached  directly  to  the  camera,  or  if  it 
must  be  so  large  as  to  cause  danger  by  vibration,  it  may  be 
connected  through  a  flexible  shaft.  This  use  of  an  electric 
motor  is  very  practical  for  semi-automatic  cameras  such  as 
the  "L"  or  the  American  deRam,  in  planes  supplied  with  a 
suitable  source  of  current. 

When  it  comes  to  entirely  automatic  cameras,  where 
uniform  and  regulatable  speed  is  required,  as  in  making 
overlapping  pictures  for  mapping,  the  electrical  drive  is  not 
so  conveneint.  The  shunt- wound  motor  runs  at  nearly 
constant  speed,  while  the  series-wound  motor  in  which  the 
speed  can  be  regulated  by  the  interposition  of  resistance, 
has  nothing  like  a  sufficient  range  of  variation  for  the  pur- 
pose (at  least  five  to  one  is  imperative)  before  it  fails  to  carry 
the  load.  Hence  we  must  either  incorporate  in  the  camera 
some  mechanism  for  varying  the  interval  between  exposures 
while  the  speed  of  the  motor  remains  constant,  or  introduce 
an  auxiliary  device  to  effect  the  required  transformation  in 
speed.  If  we  do  use  an  auxiliary  device  the  train  of  appara- 
tus, consisting  of  battery  (or  generator),  motor,  speed  con- 
trol and  camera,  is  altogether  too  long;  it  is  apt  to  cause 


152         AIRPLANE  PHOTOGRAPHY 

annoying  delays  in  connecting  up  in  an  emergency,  and  it 
offers  an  excessive  number  of  chances  for  break-down.  ' 

Performance  and  Efficiency  Data. — The  first  step  in 
deciding  upon  methods  of  power  drive,  and  indeed  in  decid- 
ing whether  power  drive  is  feasible  at  all,  is  to  assemble 
definite  data  as  to  the  power  required  to  drive  representative 
cameras.  Approximate  figures  for  some  of  the  cameras 
described  in  previous  chapters  are: 

L  camera, 26  watts, 

deRam, 60  watts, 

"K"  film, 30  watts. 

These  requirements — not  exceeding  1/10  horse  power — are 
insignificant  in  comparison  with  the  total  of  100  to  400 
horse  power  available  for  all  purposes  from  the  plane's  engine. 

Propeller  characteristics.  Data  on  the  performance  of 
small  propellers  are  somewhat  meagre.  However,  the  results 
of  the  rather  extensive  researches  on  large  ones,  suitable  for 
driving  planes,  may  be  applied,  with  proper  reservations,  to 
give  a  fair  guide  to  the  study  of  the  application  of  small 
propellers  for  driving  plane  auxiliaries. 

The  first  factor  to  be  considered  is  the  thrust  or  head 
resistance  offered  by  a  propeller  to  motion  through  the  air. 
This  varies  as  the  square  of  the  velocity,  as  the  density  of  the 
medium,  and  as  the  area  of  the  body  projected  normally  to  the 
wind,  the  formula  being 

T=cdaV* 

where  T  —  thrust,  d=  density,  a=area,  V=  velocity.  Data 
on  the  L  camera  propeller  are  shown  in  Fig.  66,  where  its 
thrust  both  when  free  and  when  loaded  with  the  camera  is 
given,  as  well  as  that  of  a  solid  disc  of  the  same  diameter  as 
the  propeller.  For  this  propeller,  which  is  double-bladed, 
and  six  inches  in  diameter,  cda  =  . 000275  with  the  load  on. 


MOTIVE  POWER 


153 


The  total  thrust  amounts  to  only  about  three  pounds  when 
the  plane  velocity  is  100  miles  per  hour.  The  head  resistance 
of  the  whole  plane  is  a  matter  of  hundreds  of  pounds,  so  that 
the  propeller  resistance  is  quite  negligible. 

The  next  factor  is  the  speed  of  revolution  of  the  propeller, 
expressed  in  revolutions  per  minute.     This  varies  with  the 


100 

vit      u.     f     l\l/»    L.  UL_  L  1  > 

1 

/    / 

'/ 

X  80 

w. 

// 

^* 

— 

1 

1 

W1 

** 

i?^ 

M 

!« 

>- 

/J 
3Lfi 

"ff. 

^&* 

^ 

AV/ 

£ 

s' 

h- 

V 

s^ 

§ 

'  7/ 

/ 

ui40 

'    7 

r/ 

f    fy 

> 

0 

z 

*PO 

III       / 
ill     / 

til/ 

w 

i 

5 

\                                                          •$                          4f                         5                         *>                           / 

THRUST  m 

FIG.  66.— Wind  propeller  data. 

design — the  number  of  blades,  their  area,  and  pitch.  For  a 
given  design  the  speed  of  revolution  is  directly  proportional 
to  the  speed  of  motion  through  the  air,  and  to  the  density  of  the 
air.  Representative  data  for  the  L  camera  propeller  are 
shown  in  Fig.  67.  It  will  be  noted  that  the  speed  goes  up  to 
8000  for  120  miles  per  hour  air  speed.  This  illustrates  the 
necessity  for  great  strength  to  withstand  centrifugal  force. 
Propellers  should  be  constructed  of  tough  material,  and 


154        AIRPLANE  PHOTOGRAPHY 


subjected  to  whirling  tests  up  to  speeds  considerably  in 
excess  of  any  the  plane  will  attain  in  any  maneuver.  At 
low  speeds  the  linear  relationship  fails,  as  a  critical  velocity 
is  reached  —  about  3500  r.  p.  m.  for  this  propeller  —  where  it 
refuses  to  turn. 

The  fact  that  the  speed  of  the  propeller  depends  on  the 


7000 

1 

.    CAMl 

RA    PR 

OPELLf 

R 

6000 

R 

EVOLU 

riONS 

X 

1 

i 

z 

X 

5 

r 

LXX 

X 

• 
i 

c 

5000 

3ooo 

X 

2 

X 

x 

SO  60  70  80  90 

WIND  VELOCITY    Miles  per  Hour 

FIG.  67. — Relation  between  air  speed  and  propeller  revolutions. 


100 


density  of  the  air  has  an  interesting  corollary,  wjiich  is  that 
a  propeller  adequate  at  low  altitudes  will  fail  at  high  ones. 
The  density  of  the  air  varies  with  altitude  according  to  the 
following  figures: 

At  3000  meters, 72  per  cent,  of  sea  level 

5000  meters, 59  per  cent,  of  sea  level 

6000  meters, .  52  per  cent,  of  sea  level 

If  we  take  the  r.  p.  m.  at  90  miles  per  hour  at  sea  level  as 
6000,  then  at  the  above  altitudes  the  speeds  will  be  4300, 


MOTIVE  POWER  155 

3500,  and  3000,  respectively.  The  last  figure  is  below  that 
for  which  this  size  of  propeller  stalls  with  its  normal  load, 
as  noted  in  the  last  paragraph.  Consequently,  if  flying  is  to 
be  done  at  these  altitudes  a  larger  propeller  must  be  carried, 
which  will  still  deliver  enough  power  at  the  lower  density. 

The  next  factor  to  be  considered  is  the  power  furnished 
by  the  propeller.  As  a  representative  figure  may  be  quoted 
the  performance  of  the  L  propeller.  This  gives  27  watts  at 
3600  revolutions  per  minute  (56  miles  per  hour).  From  this 
figure  the  performance  of  other  propellers  may  be  deduced 
from  the  basic  laws,  which  are :  that  the  power  varies  as  the 
density  of  the  medium  and  as  the  cube  of  the  velocity  (assuming 
constant  efficiency).  Since  the  power  delivered  by  the  six 
inch  diameter  L  propeller  is  already  adequate  at  60  miles  per 
hour,  the  necessary  dimension  to  function  satisfactorily  at 
100  miles  per  hour  would  need  to  be  only  a  little  more  than 
three  inches,  except  for  the  desirability  of  a  safety  factor  for 
high  altitudes  and  low  air  densities. 

The  efficiency  of  the  propeller  is  defined  by  the  relation — 

Efficiency =P^er  d*"  by  **  P"**""* 
power  supplied  to  the  propeller 

The  denominator  of  this  fraction  is  the  thrust  times  the 
velocity,  for  which  the  curves  of  Fig.  66  supply  us  data  for 
the  L  propeller.  Using  the  figures  3600  r.  p.  m.,  56  miles 
per  hour,  and  27  watts,  we  find  the  efficiency  to  be  about  50 
per  cent.  This  increases  with  the  velocity,  with  a  possible 
upper  limit  of  70  to  80  per  cent.  Since  the  main  propeller 
of  the  plane  is  not  over  80  per  cent,  efficient  we  have  almost 
an  efficiency  of  64  per  cent,  in  using  a  propeller  drive,  as 
compared  with  taking  the  power  directly  off  the  engine. 

In  considering  the  use  of  spring  and  clock-work  motors  we 
meet  at  once  with  the  problem  of  comparing  the  effect  on  the 
performance  of  a  plane  of  a  carried  weight,  as  against  a 


156         AIRPLANE  PHOTOGRAPHY 

head  resistance.  The  efficiency  of  a  spring  motor  is  measured 
in  terms  of  its  weight,  that  of  a  propeller  in  terms  of  its  head 
resistance.  The  general  answer  to  this  question  is  given  by 
the  relation  that  a  pound  of  dead  weight  is  equivalent  to  % 
pound  head  resistance. 

In  order  to  apply  this  relation  to  the  study  of  spring 
motors  for  driving  cameras,  data  are  necessary  on  the  power 
delivery  per  pound  weight  of  such  mechanisms.  Such  data 
are  not  easily  accessible,  largely  because  clock-work  has  not 
generally  been  seriously  considered  as  a  motive  power  for 
large  apparatus.  To  arrive  at  an  approximate  figure  we 
may  take  the  fact  that  in  an  8  X 10  inch  film  camera  designed 
by  one  of  the  manufacturers  who  have  utilized  clock-work, 
the  motor  weighed  30  pounds.  This  is  equivalent  to  six 
pounds  head  resistance.  Now  the  type  K,  18  X24  centimeter 
film  camera  is  operated,  even  with  the  addition  of  a  friction 
drive  speed  control,  by  means  of  the  L  camera  propeller.  As 
shown  in  Fig.  66,  at  100  miles  per  hour  the  head  resistance 
of  this  propeller  is  still  less  than  three  pounds.  Consequently, 
it  appears  that  from  the  efficiency  standpoint  the  clock 
mechanism  is  quite  outclassed  by  the  wind  propeller. 

Coming  next  to  the  electric  motors,  the  L  camera  and  the 
K  are  both  operated  satisfactorily  with  a  J^o  horse  power 
motor,  weighing  6  pounds.  For  the  deRam  a  Jf  Q  horse  power 
motor  has  been  adopted. 

Taking  up  efficiency  considerations,  we  have,  if  the  cur- 
rent is  supplied  by  a  generator  from  the  engine,  a  transfor- 
mation factor  of  70  to  80  per  cent,  from  mechanical  to  elec- 
trical energy  and  a  similar  factor  in  using  a  motor  for  the 
camera.  When  batteries  are  employed  the  matter  of  weight 
versus  head  resistance  again  arises.  The  batteries  found 
most  satisfactory  for  operating  the  K  and  deRam  cameras 
are  of  the  six-cell  12  volt  lead  type.  Their  capacity  is  40 


MOTIVE  POWER  157 

ampere  hours  at  three  amperes  or  36  at  five  amperes — more 
than  is  necessary  for  a  single  reconnaissance,  but  a  practical 
figure  when  economy  of  charging  and  replacement  are  con- 
sidered. The  weight  of  this  unit  is  27  pounds.  To  this  must 
be  added  the  weight  of  the  motor — 6  Ibs. — making  a  total 
of  33  pounds,  equivalent  to  a  head  resistance  of  nearly  7 
pounds.  This  is  more  than  twice  the  propeller  head  resist- 
ance invoked  to  do  the  same  work. 

These  considerations  of  efficiency  have  been  gone  into 
because  they  are  usual  in  studying  any  engineering  problem 
and  because  of  the  insistent  demand  from  the  plane  designer 
that  every  ounce  of  weight  and  head  resistance  be  saved. 
Actually,  as  already  stated,  the  load  imposed  by  any  method 
of  power  drive  is  trivial  in  comparison  with  the  whole  load 
of  the  plane.  There  is,  however,  an  important  reservation  to 
be  made,  which  applies  against  clock-work  and  batteries: 
This  is,  that  while  the  equivalent  head  resistance  of  any 
camera  motive  power  carried  as  dead  weight  is  small,  its 
effect  on  balance  may  not  be  so.  While  the  use  of  a  propeller 
need  not  disturb  the  plane's  balance,  the  weight  of  the  camera 
alone,  without  any  driving  apparatus,  is  already  seriously 
objected  to  on  this  score.  The  merely  mechanical  superiority 
of  the  propeller  as  a  source  of  motive  power  is  on  the  whole 
rather  marked. 

Control  of  Camera  Speed. — In  the  semi-automatic 
camera  the  only  control  required  on  the  speed  of  the  operat- 
ing motor  is  at  the  upper  and  lower  limits.  It  must  not  go 
so  fast  as  to  anticipate  the  completion  of  any  steps  in  the 
cycle  of  camera  operation,  such  as  the  fall  of  plates  or  pawls 
into  position,  which  would  jam  the  camera.  On  the  other 
hand, it  must  not  be  so  slow  that  pictures  cannot  be  obtained 
with  the  requisite  overlap  for  maps  or  stereoscopic  views. 
In  the  American  deRam  camera  the  cycle  of  operations  can- 


158        AIRPLANE  PHOTOGRAPHY 

not  safely  be  put  through  in  less  than  four  seconds,  a  short 
enough  interval  for  most  purposes.  It  is  also  highly  desirable 
in  the  semi-automatic  camera  to  have  the  motive  power 
capable  of  stopping  completely.  This  saves  wear  and  tear 
on  both  motor  and  camera  mechanism. 

In  the  automatic  camera  an  extreme  range  of  speed  is 
called  for  by  the  several  problems  of  mapping,  oblique 
photography,  and  the  making  of  stereoscopic  views.  For 
mapping  alone,  the  shortest  likely  interval  may  be  taken  as 
that  required  for  work  at  approximately  1000  meters  alti- 
tude, for  a  plane  speed  of  150  kilometers  per  hour,  which 
demands  an  interval  of  six  seconds  with  a  ten  inch  lens  on  a 
4X5  inch  plate.  For  vertical  stereos  at  the  same  altitude 
and  speed  this  interval  is  divided  by  three,  and  low  oblique 
stereos  need  even  quicker  operation.  Hence  a  range  of  from 
1  to  30  pictures  per  minute  should  be  provided  for.  This 
requirement  is  difficult  to  meet  with  any  simple  mechanism. 

From  the  standpoint  of  simplicity  in  speed  regulation  the 
wind  turbine  of  adequate  vane  surface  has  much  to  recom- 
mend it.  It  is  only  necessary  to  present  more  or  less  of  its 
vane  area  to  the  wind  in  order  to  secure  a  considerable  range 
of  speed.  The  method  of  doing  this  by  a  shutter  interposed 
in  front  is  uneconomical,  but  it  is  probable  that  the  design 
can  be  so  altered  that  more  or  less  of  the  turbine  is  exposed 
beyond  the  side  of  the  plane,  possibly  by  varying  the  angle, 
to  secure  the  same  result  without  introducing  useless  head 
resistance.  A  serious  practical  objection  to  the  turbine  lies 
in  the  large  vane  surface  necessary  to  give  adequate  power 
combined  with  proper  speed  variation.  In  the  automatic 
film  camera  (Type  K)  this  area  should  be  as  much  as  40  to 
50  square  inches. 

The  wind  propeller  does  not  lend  itself  at  all  well  to  speed 
variation.  It  cannot  be  partially  covered  from  the  air  stream, 


MOTIVE  POWER  159 

as  can  the  turbine,  because  of  the  resulting  strain  on  its 
mount.  A  possible  form  of  variable  speed  propeller,  one 
which,  however,  has  not  yet  been  practically  developed,  is  a 
propeller  with  controllable  variable  pitch.  If  this  could  be 
made  mechanically  sound  it  would  be  well-suited  for  camera 
operation.  That  such  a  propeller  could  be  worked  out  is 
indicated  by  the  good  performance  of  a  constant  speed 
propeller  developed  for  radio  generators  and  used  on  the 
French  deRam  camera  (Fig.  54).  Parenthetically, it  maybe 
questioned  whether  a  constant  speed  propeller  is  really  de- 
sirable with  an  airplane  camera.  What  is  required  is  not 
exposures  at  a  definite  time  interval — although  most  of  the 
data  are  in  that  form — but  exposures  at  definite  intervals 
with  respect  to  the  motion  of  the  plane,  which  practically 
means  with  reference  to  its  air  speed.  Rather  than  build  a 
camera  calculated  to  give  exposures  at  intervals  of  so  many 
seconds  when  it  is  attached  to  a  constant  speed  propeller, 
we  would  do  better  to  use  a  propeller  which  responds  to  the 
speed  of  the  plane,  in  conjunction  with  some  form  of  tacho- 
meter to  show  the  rate  at  which  exposures  are  being  made. 
This  in  turn  should  be  coordinated  with  the  indications  of  a 
proper  camera-field  indicating  sight. 

One  solution  of  the  problem  of  speed  control  with  a  pro- 
peller of  practically  fixed  speed,  is  to  use  a  governor  and  slip 
clutch  as  in  the  English  Type  F  film  camera  (Fig.  57). 
Here  the  propeller  shaft  and  the  camera  driving  axle  are 
connected  by  two  friction  discs.  That  on  the  camera  mechan- 
ism is  forced  against  the  other  by  a  spiral  spring,  whose  tension 
is  controlled  by  a  ball  governor.  If  the  camera  speed  becomes 
too  high  the  governor  reduces  the  tension  on  the  spiral  spring 
and  the  discs  slip  over  each  other.  The  point  where  this  slip- 
ping occurs  is  determined  by  the  position  of  the  governor  as  a 
whole,  and  this  is  controlled  by  a  lever  on  top  of  the  camera. 


160 


AIRPLANE  PHOTOGRAPHY 


Another  speed  control  device,  perhaps  more  positive  but 
certainly  more  complicated  and  wasteful  of  power,  consists 
of  a  large  flat  disc,  driven  by  the  propeller  or  electric  motor, 
and  from  which  the  camera  is  driven  by  a  shaft  from  a  smaller 
friction  disc  which  may  be  pressed  against  any  point  from 


FIG.  68. — Friction  disc  speed  control. 

the  center  to  the  periphery  of  the  larger  disc.  The  speed 
range  attainable  in  this  way  is  limited  only  by  the  size  of 
the  large  disc.  An  application  of  this  idea  is  shown  in  the 
speed  control  (Fig.  68),  designed  for  the  American  Type  K 
camera  when  operated  on  an  electric  motor  or  on  a  simple 
propeller.  The  same  idea  is  utilized  in  the  Duchatellier  film 
camera,  in  connection  with  the  constant  speed  propeller 
already  described. 


MOTIVE  POWER  161 

On  the  whole  it  is  eminently  desirable  from  the  stand- 
point of  power  operation  that  the  automatic  camera  should 
embody  its  own  means  for  altering  the  interval  between 
exposures,  so  that  all  the  external  attachment  needed  is  a 
single  connection  to  a  source  of  power  either  of  constant 
speed,  as  an  electric  motor,  or  of  speed  proportional  to  that 
of  the  plane,  as  with  a  simple  wind  propeller.  This  makes  the 
camera  largely  independent  of  the  nature  of  the  power  supply, 
whereas  a  camera  designed  for  a  special  variable  speed  device 
is  of  little  use  on  a  plane  where  this  is  not  available. 

Transmission  of  Power  to  the  Camera — It  has  already 
been  pointed  out  that  the  ease  of  transmission  of  electrical 
energy  makes  it  particularly  convenient  for  use  in  a  plane. 
All  other  sources  of  power,  except  clock-work  incorporated 
in  the  camera,  require  flexible  shafting,  so  that  the  question 
of  bearings  and  connections  becomes  a  serious  one,  especially 
when  the  shaft  runs  continuously  for  long  periods  at  very 
high  speeds. 

The  shafting  found  most  suitable  is  the  spirally  wound 
form  commonly  known  as  dental  shafting.  This  must  be 
encased  in  a  smoothly  fitting  sheath,  flexible  enough  to  per- 
mit of  easy  bends.  The  ends  of  the  shaft  should  be  equipped 
with  square  or  rectangular  pins  to  fit  into  corresponding  slots 
in  the  motor  and  camera  shafts.  The  ends  of  the  shaft  casing 
may  be  fitted  either  to  attach  by  bayonet  joints  or  by 
smoothly  fitting  screw  collars.  At  the  point  of  attachment 
to  the  camera  it  is  desirable  to  have  some  form  of  junction 
adjustable  as  to  the  direction  from  which  the  shaft  may  be 
connected,  so  that  it  need  be  bent  as  little  as  possible.  A 
right  angle  bevel  gear  offers  one  means  of  doing  this.  Bear- 
ings, such  as  those  of  the  propeller,  should  be  of  the  ball 
variety,  while  heavy  lubrication,  such  as  vaseline,  should  be 
freely  used,  both  in  the  bearings  and  in  the  shaft  casing. 
11 


162         AIRPLANE  PHOTOGRAPHY 

An  important  feature  of  any  power  drive  system  should 
be  a  safety  device,  so  that  the  power  will  race  in  case  of  any 
jam  or  stoppage  in  the  camera.  This  will  often  prevent 
serious  damage  through  the  breakage  of  some  relatively 
weak  part  of  the  camera  mechanism  on  which  the  whole  force 
of  the  driving  apparatus  is  suddenly  thrown.  The  "L" 
camera  propeller  is  fitted  with  a  spring  friction  clutch  wTith 
the  idea  that  if  the  camera  refuses  to  operate  the  propeller 
will  slip  instead  of  wrenching  the  shaft  to  pieces. 


CHAPTER  XIII 
CAMERA  AUXILIARIES 

Distance  Controls  and  Indicators. — All  operations  con- 
nected with  the  exposing  and  changing  of  plates  (except  the 
changing  of  whole  magazines)  should  be  arranged  for  accom- 
plishment at  a  distance.  Other  operations,  such  as  changing 
the  shutter  speed  or  the  interval  between  exposures  in  an 
automatic  camera,  which  are  usually  done  on  the  ground, 
may  sometimes  be  satisfactorily  left  for  performance  at  the 
camera.  Conditions  of  extreme  inaccessibility  may,  however, 
make  it  necessary  to  carry  even  these  controls  to  a  distance. 
Indicators  of  the  number  of  exposures  already  made,  and  of 
the  readiness  of  the  camera  for  the  next  exposure,  may  be 
attached  to  the  camera,  but  often  are  more  profitably  placed 
at  a  distance.  Distance  control  and  indication  are  especially 
necessary  if  the  pilot  makes  the  exposures — a  common 
English  practice  in  two  seaters,  and  the  only  recourse  in 
single  seaters. 

When  electric  power  is  available,  electrical  distance  con- 
trol devices  are  perhaps  the  simplest  kind,  as  they  transmit 
motive  power  without  displacing  or  jarring  the  camera. 
Solenoids  suffice  for  the  simple  pressing  of  releases  or  for 
counting  mechanisms,  while  small  service  motors  may  be 
utilized  for  operations  involving  more  work.  A  standing 
practical  objection  to  electrical  control  lies  in  the  necessity 
for  using  contacts,  which  are  apt  to  be  uncertain  under 
conditions  that  involve  vibration. 

The  Bowden  wire — a  wire  cable  carried  inside  a  heavy  non- 
extensible  but  flexible  sheath — constitutes  the  most  satis- 
factory mechanical  means  for  transmitting  straight  pulls. 

163 


164        AIRPLANE  PHOTOGRAPHY 

By  means  of  "the  Bowden"  a  pull  may  be  transmitted  so  as 
to  be  made  entirely  relative  to  two  parts  of  the  same  body, 
calling  forth  no  tendency  of  the  body  as  a  whole  to  move. 
Thus  in  the  L  camera  shutter  release  (Fig.  50),  the  releasing 
lever  with  its  attached  counter  is  several  feet  distant  from 
the  camera.  If  the  plate  bearing  the  lever  and  sheath  end 
is  rigidly  fastened  down,  the  pressure  exerted  on  moving  the 
lever  acts  between  the  lever  and  the  end  of  the  sheath.  This 
pressure  passes  immediately  to  the  other  end  of  the  sheath, 
while  the  pull  on  the  wire  is  transmitted  to  its  farther  end  on 
the  camera.  In  this  way  the  conditions  at  the  lever  are 
reproduced,  but  with  the  advantage  that,  due  to  the  flexible 
cable  and  sheath,  any  vibration  of  the  lever  support  is 
damped  out. 

Due  to  its  stretching,  there  is  a  pretty  definite  limitation 
to  the  feasible  length  of  the  Bowden  wire.  This  length  is 
about  four  feet.  Where  according  to  English  practice  the 
pilot  makes  the  exposure,  a  considerably  longer  wire  and 
sheath  are  called  for.  In  this  case  the  effective  length  of  the 
release  is  increased  by  giving  the  pilot  a  second  releasing 
lever,  connected  to  the  first  by  a  rigid  rod  (Fig.  69).  The 
releasing  lever,  wire,  and  all  mechanical  parts  of  the  BowTden 
release  should  be  made  much  stronger  than  would  be  indi- 
cated by  bench  tests  of  the  camera.  In  the  air  it  is  impos- 
sible to  decide  either  by  sound  or  by  delicacy  of  touch 
whether  the  mechanism  has  acted,  so  that  the  observer  is  apt 
to  pull  much  harder  than  necessary  and  to  strain  or  break 
the  release  if  it  is  weak. 

The  Bowden  wire  is  used  in  the  American  service  only 
for  shutter  release.  In  the  English  service  it  has  been  used 
for  plate  changing  with  the  L  camera. 

Sights. — In  airplane  photography  the  need  for  a  finder 
or  sight  is  fully  as  great  as  in  everyday  work.  A  new  condi- 


166         AIRPLANE  PHOTOGRAPHY 

tion,  however,  prevails,  for  except  with  hand-held  cameras, 
and  even  to  some  extent  with  them,  the  operation  of  pointing 
the  camera  involves  pointing  the  whole  vehicle  that  carries 
the  camera.  The  pointing  of  airplane  cameras  is  therefore 
akin  to  the  sighting  of  great  guns.  While  the  observer  may 
perform  the  actual  operation  of  taking  the  picture,  the 
responsibility  for  covering  the  objective  rests  with  the  pilot. 
Teamwork  counts  equally  with  tools.  Airplane  camera  sights 
may  accordingly  be  divided  into  two  classes:  sights  attached 
to  the  camera,  for  use  principally  with  hand-held  apparatus, 
and  sights  attached  to  the  plane,  for  the  use  of  pilot,  of 
observer,  or  of  both. 

Sights  for  Hand=held  Cameras. — The  simplest  form  of 
sight  attached  directly  to  the  camera  is  modeled  on  the  gun 
sight,  consisting  of  a  forward  point  or  bead  and  a  rear  V. 
This,  sight  of  course  serves  merely  to  place  the  objective  in 
the  center  of  the  plate  and  gives  no  indication  of  the  size 
of  field  covered.  Another  simple  sight  of  rather  better  type 
is  the  tube  sight — a  metal  tube  of  approximately  one  inch 
diameter  and  three  inches  length,  carrying  at  each  end  pairs 
of  wires  crossed  at  right  angles.  The  camera  is  in  alignment 
when  the  front  and  back  cross  wires  both  exactly  match  on 
the  object  to  be  photographed.  The  best  way  to  mount  the 
cross-wires  is  with  one  pair  turned  through  45  degrees  with 
respect  to  the  other,  so  that  it  is  at  once  apparent  which  is 
the  front  and  which  the  rear  pair  (Figs.  31  and  39). 

Sights  to  indicate  the  size  of  the  field  are  usually  less  needed 
on  hand  cameras  than  on  fixed  vertical  cameras.  Yet  certain 
circumstances  make  them  most  desirable,  for  instance  in 
naval  work  where  a  complete  convoy  must  be  included  on 
the  plate.  A  sight  of  this  kind  can  be  made  up  of  two  wire 
or  stamped  metal  rectangles,  a  large  one  in  front  and  a 
smaller  one  behind,  of  such  relative  sizes  and  separations 


CAMERA  AUXILIARIES'  167 

that  the  true  camera  field  is  outlined  when  the  eye  is  placed 
in  position  to  see  the  two  rectangles  just  cover  each  other. 
The  dimensions  should  be  so  chosen  that  the  correct  position 
of  the  eye  is  approximately  its  natural  location  with  respect 
to  the  camera  when  this  is  held  in  the  hands  in  the  plane. 
It  is  usual  to  provide  the  rectangular  sights  with  cross-wires 
to  indicate  the  center  of  the  field.  Alternative  rear  sights 
are  simple  beads  or  peep-holes — the  use  of  the  bead  assuming 
that  the  camera  is  held  at  about  the  right  distance  from  the 
eye  for  the  rectangle  to  indicate  the  field.  The  peep-sight 
is  not  a  desirable  form,  as  it  is  hard  to  hold  the  camera  as 
near  the  face  as  is  necessary.  These  various  types  of  rec- 
tangle sights  are  well  illustrated  in  the  cameras  shown  in 
Figs.  38,  40  and  186.  They  are  all  made  so  as  to  fold  down 
flat  on  the  camera  and  to  snap  quickly  open  when  needed. 
The  springs  to  support  the  sights  must  be  fairly  strong,  and 
the  surface  presented  to  the  wind  as  small  as  possible.  Wire 
frames  give  very  little  from  the  pressure  of  the  wind,  but 
flat  metal  frames  are  apt  to  be  bent  back. 

The  position  of  the  sight  on  the  camera  is  important.  If 
the  observer  can  stand,  or  if  he  sits  up  well  above  the  edge 
of  the  cockpit,  the  conventional  position  of  the  sight  on  a 
pistol,  namely,  on  top,  is  unobjectionable.  But  if  the  ob- 
server sits  very  low,  as  he  usually  does,  then  the  sight  should 
be  on  the  bottom  of  the  camera,  thereby  avoiding  any  need 
for  the  observer  to  raise  his  head  unduly  into  the  slip  stream. 
Similarly,  if  the  camera  is  used  over  the  side  for  verticals, 
as  it  is  in  flying  boats,  a  sight  on  the  top  is  impractical,  since 
it  requires  the  observer  to  lean  out  dangerously  far  (Fig.  185). 

Sights  Attached  to  the  Plane. — Any  of  the  sights  just 
described  can  be  attached  to  cameras  fixed  in  the  plane,  but 
they  would  be  useless  in  the  positions  ordinarily  occupied 
by  the  camera.  It  has  therefore  become  common  practice 


168 


AIRPLANE  PHOTOGRAPHY 


to  attach  the  camera  sight  to  some  accessible  part  of  the 
plane.  The  most  primitive  method  of  sighting  is  merely  to 
look  downward  over  the  side — a  method  in  general  use  to 
the  very  end  of  the  Great  War.  One  step  in  advance  of  this 

is  to  mark  a  large  inverted  V 
on  the  side,  with  its  vertex  at  a 
point  where  the  observer  can 
place  his  eye  and  so  see  the 
fore  and  aft  extension  of  the 
field  of  view  covered  by  the 
camera.  This  kind  of  sight 
was  common  on  the  French 
"photo"  planes.  On  some  of 
the  English  planes  the  tube 
sight  was  carried  on  the  out- 
side of  the  cockpit.  Any  of 
the  sights  described  can  be 
carried  on  the  inside  of  the 
fuselage,  provided  a  hole  is 
cut  in  the  floor.  For  satisfac- 
tory sighting  a  hole  in  the 
floor  is  really  necessary,  as  it 
enables  the  terrain  on  both 
sides  of  the  vertical  to  be 
seen.  One  drawback  to  the 
simple  hole,  however,  is  that  it 
cannot  be  made  large  enough 
to  show  the  whole  field  from 
the  ordinary  height  of  the  observer's  eye,  thus  forcing  him 
to  bring  his  head  down  near  the  floor.  This  difficulty  is 
gotten  over  in  a  very  beautiful  way  by  the  use  of  the  negative 
lens  sight  shown  diagrammatically  in  Fig.  71. 

Let  FI  be  the  distance  at  which  the  edge  of  the  hole  (or  a 


FIG.  71. — Diagram  of  negative  lens  sight. 


CAMERA  AUXILIARIES'  169 

rectangle  marked  on  the  lens)  appears  the  size  of  the  camera 
field  (if  the  hole  is  the  size  of  the  plate,  FI  is  the  focal  length 
of  the  camera  lens).  Let  FZ  be  the  distance  from  the  floor 
to  the  observer's  eye.  What  is  desired  is  a  concave  lens 
which  will  diverge  the  rays  from  their  normal  meeting  point 
at  FI  to  a  new  meeting  point,  F2.  The  focal  length  of  lens 
required  is  given  at  once  by  the  simple  lens  formula — 

-L_-l=  JL 
Fl      F,       F 

Thus  if  FI  is  12  inches,  and  F2  is  36  inches,  F  will  be  18  inches. 
The  lens  is  to  be  marked  with  a  rectangle  showing  the  shape 
and  size  of  the  camera  field,  and  a  central  mark  such  as  a 
cross.  An  upper  rectangle,  or  a  bead,  or  a  pair  of  cross  wires 
a  few  inches  below  the  lens,  may  be  used  for  the  other  sight. 
For  precision  work  the  sight  above  or  below  the  lens  should 
be  adjustable  in  position,  especially  where  the  camera  sus- 
pension permits  the  camera  to  be  adjusted  for  the  angle  of 
incidence  of  the  plane. 

A  negative  lens  sight  should  be  placed  in  the  observer's 
cockpit,  if  he  takes  the  pictures,  and  also  in  the  forward  cock- 
pit, so  that  the  pilot  may  be  accurately  guided  in  his  part  of 
the  task.  In  addition,  it  is  advisable  to  place  a  negative  lens 
well  forward  in  the  pilot's  cockpit,  to  enable  him  to  see  the 
country  some  distance  ahead.  The  lenses  should  be  plano- 
concave with  the  flat  side  upward;  otherwise,  all  the  loose 
dirt  in  the  airplane  settles  in  the  middle  of  the  concave  de- 
pression. A  negative  lens  sight  in  a  metal  frame  forming  a 
completely  self-contained  unit  ready  for  mounting  in  the 
plane  is  shown  in  Figs.  72  and  73. 

Devices  for  Recording  Data  on  Plates. — Numbering 
devices.  The  number  of  the  camera  is  impressed  on  negatives 
taken  with  the  American  L  camera  through  the  agency  of  a 


170         AIRPLANE  PHOTOGRAPHY 

small  transparent  corner  of  celluloid.  It  would  be  entirely 
possible  to  incorporate  a  rotating  disc  which  should  turn  by 
the  operation  of  plate  changing  and  carry  a  series  of  numbers, 
so  that  each  exposure  could  be  numbered  serially.  Number- 
ing of  individual  plates  is  more  commonly  done  by  holes, 
notches,  or  even  numerals,  hi  the  turned  over  portion  of  the 
sheaths,  which  are  then  recorded  photographically  when  a 
picture  is  taken  (Fig.  75).  The  chief  objection  to  this  method 


FIG.  72. — Negative  lens  and  mount,  viewed  from  above. 

is  the  difficulty  of  keeping  the  sheaths  together  in  sets, 
especially  as  individual  ones  become  damaged  or  lost.  In 
practice  there  is  also  danger  of  the  sheaths  being  carelessly 
loaded  in  wrong  order. 

The  more  ambitious  idea  of  recording  on  the  plate  all  the 
information  given  by  the  instrument  board  of  the  plane 
occurs  independently  and  spontaneously  to  all  aerial  photo- 
graphic map  makers.  These  ideas  vary  from  attempts  to 
photograph  the  actual  instrument  board  on  every  plate— a 
difficult  task  indeed  with  the  instruments  and  camera  placed 


CAMERA  AUXILIARIES 


171 


as  they  are  in  the  ordinary  plane — to  the  incorporation  of 
compass,  altimeter,  and  inclinometer  in  the  camera  itself. 

Figure  58  shows  the  plan  adopted  in  the  English  F  type 
film  mapping  camera  already  described,  for  photographing  a 
compass  and  an  altimeter  on  the  film.  Here  the  combined 
compass  and  altimeter  dial  is  above  the  camera,  and  is 


FIG.  73. — Negative  lens  and  mount,  side  view. 

mounted  in  a  cell  with  a  glass  bottom.  Below  it  is  a  lens 
focussing  the  needles  and  compass  points  on  the  plane  of  the. 
film.  The  light  for  photography  is  furnished  by  a  diffusely 
reflecting  white  surface  on  top  of  the  camera,  illuminated 
by  the  sky.  (The  camera  was  carried  outboard.)  In  Fig.  56 
is  shown  a  picture  with  the  compass  image  impressed  upon  it. 
Figure  74  shows  a  type  of  inclination  indicator  found  in 
some  captured  German  cameras.  It  consists  essentially  of 


172         AIRPLANE  PHOTOGRAPHY 

Section 


/ncf/nometer  n 


/i/e  Plate 

Assembk 


FIQ.  74r. — Diagram  of  inclinometer  used  in  some  German  cameras. 


CAMERA  AUXILIARIES 


173 


two  small  pendulums  or  plumb-bobs;  one  to  indicate  lateral, 
the  other  longitudinal  inclination,  arranged  to  be  photo- 
graphed in  silhouette  on  the  plate,  as  shown  in  the  lower 
part  of  the  diagram  and  in  the  print  from  a  captured  nega- 
tive (Fig.  75). 


FIG.  75. — Photograph  made  with  German  camera,  showing  inclinometer  record,  four  points    for 
locating  diameters  and  center  of  plate,  and  (upper  right-hand  corner)  number  of  the  plate  sheath. 

Both  these  devices  suffer  from  the  deficiencies  of  the 
instruments  they  photograph.  The  compass  and  the  incli- 
nometer, as  already  mentioned  in  the  discussion  of  airplane 
instruments,  only  behave  normally  in  straight-away  flying, 
failing  to  indicate  correctly  when  the  plane  is  subject  to 
accelerations  in  any  direction.  In  general  all  attempts  to 
record  directional  data  in  the  camera  are  of  little  promise, 
unless  either  the  instruments  or  the  camera  are  automatically 


174        AIRPLANE  PHOTOGRAPHY 

held  level  by  some  gyroscopic  device.  If  the  instruments  are 
so  controlled,  rather  elaborate  means  for  photographing  them 
are  necessary.  If  the  camera  is  stabilized,  the  inclinometers 
are  unnecessary,  and  the  compass  behaves  rationally. 

Another  scheme  for  indicating  inclinations,  which  is  not 
subject  to  the  above  objections,  is  to  photograph  the  horizon 
either  on  a  separate  film  or  on  the  same  sensitive  surface, 
simultaneously  with  the  principal  exposure.  The  difficulty 
here  is  the  practical  one  that  it  is  only  feasible  in  localities 
of  great  atmospheric  clearness.  Ordinarily,  especially  any- 
where near  the  sea-coast,  the  horizon  is  too  rarely  seen  to  be 
a  reliable  mark  (Fig.  4).  It  is  possible,  however,  that  this 
objection  could  be  overcome  by  the  use  of  specially  red 
sensitive  plates  and  suitable  color  filters,  as  discussed  in  the 
chapter  on  "Filters."  The  method  would  in  any  case  be 
useless  in  mountainous  country. 

The  difficulties  discussed  with  reference  to  direction  indi- 
cating instruments  of  course  do  not  hold  with  the  altimeter. 
Ordinarily,  though,  the  altitude  changes  slowly  enough  to 
permit  of  sufficiently  accurate  records  being  made  by  pencil 
and  pad.  For  high  precision  map  making  a  photographic 
record  of  altimeter  readings  has  a  legitimate  claim.  As  we 
have  seen,  a  small  altimeter  is  incorporated  in  the  English  F 
camera,  but  the  bulk  which  a  really  precision  altimeter 
would  assume  would  be  a  bar  to  its  use  in  this  way.  A  time 
or  serial  number  record  on  the  plate  or  film,  synchronized 
with  a  similar  record  on  the  film  of  an  auxiliary  camera 
which  photographs  the  altimeter  and  other  instruments, 
may  be  the  simplest  way  to  preserve  the  majority  of  the 
desired  data. 

Devices  for  Heating  the  Camera. — Parts  of  the  camera 
mechanism  which  depend  on  the  uniformity  of  action  of 
springs  or  upon  adequate  lubrication  are  susceptible  to  change 


CAMERA  AUXILIARIES  175 

with  variation  of  temperature.  At  high  altitudes  low  tem- 
peratures are  met  which  may  freeze  ordinary  machine  oils 
or  may  cause  springs  to  seriously  alter  their  tension,  even  to 
break.  To  meet  this  difficulty,  and  probably  also  to  dispel 
the  occasional  condensation  of  moisture  on  the  optical  parts, 
the  German  cameras  are  equipped  with  an  electrical  heating 
coil  placed  just  below  the  shutter,  and  arranged  to  connect 
with  the  general  heating  and  lighting  current  of  the  plane. 
Two  contacts  are  ordinarily  provided,  for  offsetting  the 
effects  of  temperatures  of  —15  and  —30  degrees  centigrade. 
An  additional  function  of  this  heating  coil  is  perhaps  to 
maintain  the  sensitiveness  of  the  plates  or  film. 


Ill 

THE  SUSPENSION  AND  INSTALLATION 
OF  AIRPLANE  CAMERAS 


CHAPTER  XIV 

THEORY    AND    EXPERIMENTAL    STUDY    OF 
METHODS  OF  CAMERA  SUSPENSION 

General  Theory. — In  addition  to  the  limitation  of  ex- 
posure set  by  the  ground  speed  of  the  plane  another  limita- 
tion is  set  by  the  vibration  of  the  camera.  This  may  be 
caused  either  by  the  motor,  or  by  the  elastic  read  ions  of  the 
plane  members  to  the  strains  of  flight.  Unlike  the  move- 
ment of  the  image  due  to  the  simple  motion  of  the  plane, 
movements  due  to  vibration  may  be  eliminated  by  proper 
anti-vibrational  mounting  of  the  camera. 

The  effect  of  vibration  may  show  as  an  indistinctness  of 
the  whole  image — this  is  its  only  effect  with  a  between-the- 
lens  shutter — or  as  a  band  or  bands  of  indistinctness  parallel 
to  the  curtain  opening  (Fig.  76).  These  are  due  to  shocks 
or  short  period  vibrations  during  the  passage  of  the  focal- 
plane  shutter. 

The  obvious  remedy  for  vibration  troubles  is  to  mount 
the  camera  on  some  elastic,  heavily  damping  support,  like 
sponge  rubber  or  metal  springs.  Such  a  mounting  should, 
however,  be  designed  on  sound  principles  derived  from  a 
proper  analysis  of  the  nature  and  effect  of  the  possible 
motions  of  the  camera.  Otherwise,  the  vibrational  disturb- 
ances may  be  increased  rather  than  diminished  by  the  camera 
mount.  Such  an  analysis,  based  merely  on  general  mechan- 
ical principles,  shows  that  all  motions  of  the  camera  are 
resolvable  into  six.  These  are  three  translational  motions, 
namely,  two  at  right  angles  in  one  plane  such  as  the  hori- 
zontal, and  one  in  the  plane  at  right  angles  to  this  (vertical) ; 

179 


180        AIRPLANE  PHOTOGRAPHY 


CAMERA  SUSPENSION  181 

and  three  rotational  motions,  one  about  each  of  the  above 
directions  of  translational  motion  as  an  axis  (Fig.  77). 

Brief  consideration  will  show  that  only  the  latter — the 
rotational  motions — are  of  any  importance  when  the  small 
displacements  due  to  vibration  are  in  question.  To  illustrate 
the  negligible  effect  of  vibrations  which  merely  move  the 
camera  parallel  to  itself  in  any  direction  it  is  only  necessary 
to  imagine  the  camera  moved  parallel  to  the  ground  through  a 
large  distance,  such  as  10  centimeters.  Now  10  centimeters 
motion  of  the  camera  at  3000  meters  elevation  means,  with 
a  25  centimeter  camera  lens, 

25  1 

X  10  =  77:7:7:  centimeter 


3000  ~  1200 

motion  on  the  plate,  which  would  be  only  a  tenth  the  distance 
separable  by  a  good  lens.  If  we  reduce  this  motion  to  the 
small  fraction  of  a  centimeter  which  vibration  would  actually 
produce,  it  is  evident  that  such  vibration  is  of  absolutely 
no  importance.  Similarly,  if  we  imagine  the  camera,  under 
the  same  conditions,  moved  vertically  with  reference  to  the 
ground  by  ten  centimeters,  the  scale  of  the  picture  would 
merely  be  changed  by  ^oo'o'  or  by  10100  centimeter  on  a 
12  centimeter  plate,  again  quite  negligible. 

When  we  consider  motions  of  rotation,  however,  the  case 
is  quite  different.  If  the  camera  is  mounted  so  that  the 
effect  of  any  vibration  is  to  rotate  it  around  a  horizontal 
axis,  this  is  exactly  equivalent  to  rotating  the  beam  of  light 
from  the  lens  so  that  it  sweeps  across  the  plate.  Thus  a 
millimeter  displacement  of  the  lens  of  the  camera  with  the 
plate  remaining  fixed  gives  approximately  a  millimeter  mo- 
tion of  the  image.  Consequently,  a  rotation  producing  only 
a  fraction  of  a  millimeter's  relative  motion  of  lens  and  plate 
during  the  period  the  curtain  aperture  is  over  a  given  point 


182 


AIRPLANE  PHOTOGRAPHY 


would  cause  fatal  blurring  —  and  the  visible  vibration  of 
plane  longerons  and  cross  members  is  easily  of  half  milli- 
meter amplitude  or  more.  Reduced  to  angular  units  it  is 
easily  shown  that  a  rotation  of  one  degree  per  second  —  which 
is  quite  slow  as  plane  oscillations  go  —  is  beyond  the  limits 

of  toleration.  Translational  mo- 
tions of  large  amplitude  may  be 
allowed,  but  the  mounting  of  the 
camera  must  not  permit  these 
translations  to  be  at  all  different 
for  different  parts  of  the  camera* 
The  proper  way  to  eliminate 
vibrational  effects  is  to  devise  a 
mounting  that  will  transmit  only 
the  translational  shocks  or  that 
will  transform  the  rotational  ones 
into  translations.  Platforms 
pivoted  and  cross-linked  so  as  to 
be  free  to  move  only  parallel  to 
themselves  (described  in  the  next 
chapter)  represent  one  attempt 
to  reach  this  result.  Quite  the 
simplest  and  most  scientific  form 
of  mounting  to  achieve  this  end  is 
to  support  the  camera  solely  in  the 
plane  of  the  center  of  gravity.  The 

principle  here  involved  is  easily  grasped  if  we  note  that  when 
we  jar  a  camera  supported  above  or  below  its  center  of 
gravity,  the  effect  is  to  start  the  camera  vibrating  with  the 
center  of  gravity  oscillating  pendulum-like  about  the  point 
of  support.  The  closer  the  center  of  gravity  to  the  center 
of  support,  the  smaller  the  moment  of  the  body  about  the 
latter  point. 


Ja1 
d  three  °f  r°tati°n> 


CAMERA  SUSPENSION'        .  iss 

Experimental  Study  of  Methods  of  Camera  Support. — 
Conclusive  evidence  as  to  the  merits  of  any  system  of  camera 
mounting  can  be  obtained  only  under  conditions  that  elimi- 
nate the  effect  of  other  variables  which  may  be  equally 
efficacious  in  diminishing  the  effects  of  vibration,  but  which 
have  only  limited  application.  Very  brief  exposures — ^-$ 
second  and  less — will,  for  instance,  result  in  good  pictures 
with  almost  any  condition  of  vibration.  Hence  a  sharp 
picture  offers  no  proof  of  the  merits  of  a  camera  mounting 
unless  it  is  known  that  the  exposure  was  no  shorter  than  the 
limit  set  by  the  ground  speed  of  the  plane.  In  fact  it  may  be 
said  that  the  chief  object  of  studying  methods  of  camera 
suspension  is  to  increase  the  allowable  exposure  to  a  maxi- 
mum, thus  lengthening  the  working  hours  and  multiplying 
the  useful  working  days  for  aerial  photography. 

The  most  satisfactory  method  of  test  yet  developed  is  to 
fly  over  a  light  or  a  group  of  lights  on  the  ground  with  the 
camera  shutter  open.  In  the  first  use  of  this  method,  which 
originated  in  the  English  Service,  such  flights  were  made  at 
night,  but  later  it  was  found  that  good  results  could  be  got 
by  placing  the  lights  in  a  forest  and  making  the  tests  when 
the  sun  was  fairly  low.  One  of  the  group  of  lights  must  be 
periodically  interrupted,  at  a  known  rate,  to  furnish  the 
time  intervals. 

Some  characteristic  "trails"  obtained  by  this  method  of 
test  are  shown  in  Fig.  78.  The  first  trail  is  that  given  by  a 
camera  rigidly  fastened  to  the  fuselage.  The  second  and 
third  show  hand  camera  trails,  made  by  an  inexperienced 
and  by  an  experienced  observer,  respectively.  They  show 
by  comparison  with  the  other  figures  that  the  human  body 
is  an  excellent  block  to  vibration,  but  in  unskilled  hands  a 
poor  check  to  rapid  erratic  (probably  rotational)  motions 
of  the  camera.  The  fourth  is  the  trail  given  by  a  camera 


184        AIRPLANE  PHOTOGRAPHY 


Otct60ar«l 


46er  (r<>*£*t 


FIG.  78. — Tests  of  camera  mounting,  made  by  flying  over  a  bright  light  against  a  dark  background, 
held  in  the  band,  inexperienced  observer;  (c)  held  in  the  hand, 


(a)  Rigid  fastening  on  side  of  plane;  (6) 

experienced  observer;    (d)  camera  mounted  at  center  of  gravity 


gimbals  bedded  in  sponge  rubber. 


supported  by  gimbals  bedded  in  sponge  rubber  accurately 
in  the  plane  of  the  camera's  center  of  gravity.  Other  trails 
are  shown  in  the  next  chapter  in  connection  with  the  de- 
scription of  practical  camera  mountings.  Clearly  the  best 


CAMERA  SUSPENSION'  185 

suspension  is  that  giving  the  smallest  amplitude  of  displace- 
ment during  the  interval  of  time  covered  by  an  average 
exposure.  It  is,  in  fact,  possible  to  determine  from  these 
trails  the  permissible  exposure  for  any  assumed  permissible 
blurring  of  the  image.  The  rigid  mounting  trail  indicates 
very  bad  conditions,  calling  for  literally  instantaneous 
exposures.  The  center  of  gravity  trail,  at  the  other  extreme, 
shows  practically  no  limitation  of  exposure  in  so  far  as  vibra- 
tion is  concerned,  thus  bearing  out  the  theoretical  conditions 
above  discussed.  An  interesting  conclusion  from  these 
experiments  is  that  a  rapidly  running  motor  gives  less  harm- 
ful vibration  than  a  slow  one,  although  in  the  war  it  was  a 
common  practice  to  throttle  the  motor  before  exposing.  As 
might  be  expected,  the  greater  the  number  of  cylinders, 
the  shorter  the  period  and  the  smaller  the  amplitude  of 
the  vibration. 

Pendular  Camera  Supports. — The  design  of  the  camera 
support  may  be  approached  from  a  different  standpoint, 
namely,  with  the  aim  of  carrying  the  camera  so  that  it  will 
tend  to  hang  always  vertical.  In  mapping  this  is  of  funda- 
mental importance.  It  is,  indeed,  a  question  whether  aerial 
mapping  will  ever  be  worthy  of  ranking  as  a  precision  method 
unless  the  camera  can  be  mounted  so  that  its  pictures  are 
taken  in  the  horizontal,  undistorted  position. 

The  simplest  way  to  hold  the  camera  vertical  is  to  mount 
it  on  gimbals,  with  its  center  of  gravity  below  the  point  of 
support.  When  so  mounted  the  camera  swings  as  a  pendu- 
lum. Delicacy  of  response  to  variation  of  level  is  obtained 
by  leaving  a  considerable  distance  between  the  center  of 
gravity  and  the  center  of  support.  Oscillation  about  the 
vertical  position  is  to  be  prevented  by  some  system  of  dash 
pots  or  other  damping.  A  suspension  of  this  kind  is  furnished 
with  the  Brock  film  camera  (Fig.  60). 


186        AIRPLANE  PHOTOGRAPHY 

It  will  be  seen  at  once  that  the  relation  of  center  of 
gravity  to  center  of  support  called  for  here  is  in  direct  con- 
tradiction to  the  requirements  for  eliminating  vibration. 
Either  one  requirement  or  the  other  must  be  sacrificed,  or 
else  a  compromise  made  in  which  neither  delicate  response  to 
inclination  of  the  plane  nor  fully  satisfactory  freedom  from 
vibration  is  attained.  This  is  a  very  serious  objection  to  the 
pendular  support.  But  the  really  vital  objection  to  the 
pendular  support  is  that  it  performs  its  function  only  very 
partially.  It  is  entirely  satisfactory  only  under  conditions 
of  steady  flying,  as  in  a  uniform  climb  or  glide,  with  the  plane 
tail  or  nose  heavy,  or  in  flying  with  one  wing  down.  As 
soon  as  we  introduce  any  acceleration,  as  in  making  a  turn, 
the  camera  follows  the  plane  and  not  the  earth. 

It  is  true  that  mapping  photography  is  done  from  a  plane 
flying  as  level  as  possible,  and  that  except  under  bad  air  con- 
ditions it  holds  its  course  with  very  little  turning,  if  handled 
by  a  skilled  pilot.  Nevertheless,  a  surprisingly  small  devia- 
tion from  straight  flying  causes  quite  serious  variations  from 
the  vertical.  It  is  of  interest  to  calculate  how  large  may  be 
the  horizontal  accelerations  that  accompany  swervings 
from  a  straight  course  which  one  might  think  insignificant. 
For  instance,  consider  the  horizontal  acceleration  due  to 
a  turn  having  a  radius  of  a  kilometer  when  the  plane  is 
moving  at  100  kilometers  per  hour.  If  a  is  the  accel- 
eration, v  the  velocity  of  the  plane,  and  r  the  radius,  we 
have  from  elementary  dynamics  that 


Substituting  the  values  chosen,  we  have 


100,000*        _   w  meters 


36002  X  1000 


CAMERA  SUSPENSION  187 

TY1  f^i"  f*T*^ 

The  acceleration  of  gravity  being  9.80  --  «  —  we  have  that 

sec 

the  ratio  of  the  horizontal  acceleration  to  the  vertical  is 


This  is  the  tangent  of  the  angle  of  deviation  from  the  ver- 
tical, from  which  the  angle  turns  out  to  be  about  4rJ  degrees, 
a  very  considerable  error,  rapidly  multiplied  as  the  speed  of 
the  plane  is  increased.  It  is,  indeed,  open  to  question  whether 
the  average  deviations  from  the  vertical  are  not  apt  to  be 
less  with  the  camera  rigidly  fixed  to  the  plane,  if  guided  by 
a  skilled  pilot  who  will  hold  the  ship  level  at  the  expense 
of  "skidding"  the  slight  turns  he]  must  make  to  hold 
his  direction. 

Gyroscopic  Mountings.  —  The  ideal  support  for  the 
aerial  camera  will  undoubtedly  be  one  embodying  gyroscopic 
control  of  the  camera's  direction.  By  proper  utilization  of 
the  principles  of  the  gyroscope  it  is  to  be  expected  that  not 
only  can  the  camera  be  maintained  vertical,  but  it  may  be 
supported  anti-vibrationally  as  well.  At  the  present  time 
the  problem  of  gyroscopic  control  is  in  the  experimental 
stage,  so  that  only  the  elements  of  the  problem  and  the  pos- 
sible modes  of  solution  can  be  laid  out. 

The  gyroscope  consists  essentially  of  a  heavy  ring  or 
disc  rotating  at  a  high  speed  on  an  axis  free  to  point  in  any 
direction  (Fig.  79).  If  mounted  so  that  the  axes  of  the  sup- 
porting gimbals  pass  through  the  center  of  gravity  of  the 
rotating  disc,  the  result  is  a  neutral  gyroscope.  Its  charac- 
teristic is  that  its  axis  maintains  its  direction  fixed,  but  this 
fixity  is  with  respect  to  space  and  not  with  respect  to  the 
gravitational  vertical.  Consequently,  as  the  earth  revolves 
the  inclination  of  the  gyroscopic  axis  changes  with  respect 


188 


AIRPLANE  PHOTOGRAPHY 


to  the  earth.  In  latitude  45°  this  change  is  approximately  a 
degree  in  five  minutes.  Furthermore,  the  action  of  friction  in 
the  supports,  which  can  never  be  entirely  eliminated,  also 
acts  to  slowly  alter  the  direction  of  the  gyroscopic  axis. 
Therefore,  unless  some  erector  is  applied  even  the  gyroscope 
will  not  perform  the  task  required  of  it. 


FIG.  79. — Diagram  of  simple  gyroscope. 

Before  discussing  possible  forms  of  erectors  it  may  be 
noted  in  general,  first,  that  these  must  depend  upon  gravity; 
second,  that  such  being  the  case,  they  must  respond  to  the 
resultant  of  gravity  and  any  acceleration,  that  is,  to  the 
apparent  or  pseudo-gravity.  As  already  seen,  this  pseudo- 
gravity,  during  a  turn,  is  exactly  what  limits  the  usefulness 


CAMERA  SUSPENSION'  189 

of  the  pendular  support,  and  necessitates  recourse  to  the 
gyroscope.  The  problem  thus  becomes  one  of  making  an 
erector-gyroscope  combination  which  will  respond  to  true 
gravity  and  not  to  pseudo-gravity. 

In  general  this  problem  would  be  insoluble,  since  there  is 
no  difference  in  the  nature  of  the  acceleration  of  gravity  and 
that  due  to  centrifugal  force.  A  way  out  is  offered,  however, 
by  the  fact  that  true  gravity  acts  continuously  and  at  a 
small  angle  to  the  axis  of  the  gyro,  while  the  components 
which  cause  the  pseudo-gravity  are  of  short  duration,  liable 
to  rapid  changes  of  direction,  and,  on  a  turn,  act  at  a  large 
angle.  What  we  require,  therefore,  is  an  erector  which  will 
respond  slowly  but  surely  to  the  average  acceleration,  which 
is  downward,  but  too  sluggishly  to  be  affected  by  the  shorter 
period  accelerations  due  to  turns  or  rolls.  Slowness  of 
response  is  a  matter  of  the  erecting  forces  being  small  and  of 
the  mass  and  angular  velocity  of  the  gyro  disc  being  large. 
The  success  of  the  compromise  called  for  depends  on  the 
relative  times  taken  for  the  gyroscope  to  tilt  seriously  from 
the  true  vertical,  due  to  the  causes  above  mentioned,  and 
for  the  average  turn  or  roll.  Fortunately  the  former  is  a 
matter  of  minutes,  the  latter  of  seconds  or  at  the  worst  of 
fractions  of  a  minute.  More  than  this,  since  the  roll  or  turn 
is  apt  to  be  of  much  greater  angle  than  any  normal  deviation 
of  the  gyroscopic  axis  from  the  vertical  in  the  same  time,  we 
are  offered  the  possibility  of  seme  device  for  filtering  out  the 
deviations  which  alone  are  to  effect  the  erector.  For  instance, 
by  shunting  the  restoring  force  whenever  it  is  called  upon  to 
act  through  more  than  a  predetermined  small  angle. 

As  to  the  method  of  erecting  the  gyroscope,  its  charac- 
teristic property  must  be  kept  in  niind.  This  is  that  the 
axis  does  not  tilt  under  an  applied  force  in  the  direction  it 
would  if  the  gyro  were  not  rotating,  but  around  an  axis  at 


190        AIRPLANE  PHOTOGRAPHY 

right  angles  to  that  of  the  applied  couple.  Thus  in  Fig.  79, 
if  a  weight  is  attached  as  shown,  the  disc  does  not  incline 
downward  toward  the  weight,  around  the  axis  Y,  Y',  but 
^recesses  about  the  vertical  axis  Z,  Z'.  Some  means  is 
therefore  needed  to  translate  the  pull  which  any  gravita- 
tional control,  such  as  a  freely  swinging  pendulum,  would 
give,  into  a  pull  with  at  least  a  component  at  a  finite  angle 
to  this. 

In  the  Gray  stabilizer  several  metal  balls  are  slowly 
rotated  in  a  tray  above  the  center  of  gravity  of  the  gyroscope. 
Specially  shaped  grooves  or  compartments  limit  the  freedom 
of  motion  of  these  balls  so  that  when  the  gyro  is  inclined  the 
balls  travel  at  different  distances  from  the  center  on  the 
ascending  and  descending  sides.  By  this  scheme  a  couple  is 
produced  about  the  axis  through  the  center  and  the  low  point 
of  the  disc,  which  tilts  the  apparatus  to  the  gravitational 
vertical.  In  an  alternative  form  the  balls  are  carried  past 
the  low  point  by  their  momentum  and  are  prevented  from 
returning  by  the  walls  of  the  containing  compartment,  which 
have  meanwhile  been  advanced  by  the  rotation  of  the  erector 
as  a  whole.  The  net  result  is  to  shift  the  center  of  gravity 
of  the  system  of  balls  in  the  proper  direction  to  erect  the 
gyro.  The  rectifying  action  is  purposely  made  quite  slow 
so  that  the  displacements  of  the  balls  due  to  pseudo-gravity 
will  be  averaged  out. 

In  a  design  due  to  Lucian,  small  pendulums  work  through 
electric  contacts  to  actuate  solenoids  which  in  turn  move 
small  weights  in  the  appropriate  directions  to  give  the 
desired  tilt.  Response  is  made  fairly  quick  and  delicate, 
and  pseudo-gravity,  due  to  turns  and  rolls,  is  rendered  inop- 
erative by  the  contacts  breaking  whenever  the  pendulums 
swing  more  than  three  or  four  degrees.  This  can  only 
happen  if  they  move  too  quickly  for  the  erecting  forces  to 


CAMERA  SUSPENSION 


191 


act,  reliance  being  here  placed  on  the  characteristic  differ- 
ences of  action  in  respect  to  time  of  real  and  pseudo-gravi- 
tational forces. 

Besides  the  neutral  gyroscope  as  just  considered  there  is 
the  pendular  or  top  type,  in  which  the  center  of  gravity  is 
not  in  the  plane  of  the  supports.  In  general  this  type  depends 
on  a  couple  resulting  from  the  gravitational  pull  and  the 
inevitable  friction  of  the  supports  to  slowly  tilt  the  axis  to 
the  gravitational  vertical.  This  type  is  slower  to  respond 


FIG.  80. — Diagram  of  camera  linked  to  gyroscopic  stabilizer. 

than  the  designs  in  which  a  definite  couple  in  the  proper 
direction  is  provided  and  it  reaches  the  true  vertical  only 
through  a  circuitous  path. 

Three  methods  of  controlling  a  camera  by  a  gyroscope 
are  suggested.  One  is  to  fasten  the  gyroscope  rigidly  to  the 
camera  and  mount  the  whole  system  on  gimbals.  A  second 
is  to  mount  both  camera  and  gyro  side  by  side  on  gimbals, 
linking  the  two  so  that  the  camera  is  moved  parallel  to  the 
gyro  (Fig.  80).  A  third  method  is  to  utilize  the  gyro  to 
make  electric  contacts  to  operate  motors  which  in  turn  move 
the  camera. 

Considerable  weight  and  space  are  required  for  a  gyro- 


192        AIRPLANE  PHOTOGRAPHY 

scope  capable  of  stabilizing  a  camera.  The  rotating  disc 
should  be  about  half  the  weight  of  the  camera,  and  with  its 
mounting  may  be  expected  to  double  the  room  required  for 
the  camera  alone.  Motive  power  for  maintainng  the  gyro 
in  continuous  rotation  may  be  supplied  by  an  air  blast,  or 
the  gyro  may  be  made  up  as  an  induction  motor — the  latter 
necessitating  an  alternating  current  supply. 

In  view  of  the  space  and  weight  limitations  in  a  plane  it 
is  a  question  still  to  be  decided  whether  it  is  more  economical 
to  stabilize  the  camera  or  to  stabilize  an  inclinometer  and 
photograph  its  indications  simultaneously  with  the  release 
of  the  shutter  which  takes  the  aerial  picture. 


CHAPTER  XV 
PRACTICAL  CAMERA  MOUNTINGS 

General  Considerations. — Camera  mountings  as  used 
during  the  war  were  far  from  being  developed  on  the  basis 
of  scientific  study  or  test.  At  first  the  need  for  special  sup- 
porting apparatus  was  not  realized,  and  the  suspensions 
later  in  use  were  largely  field-made  affairs,  often  dependent 
on  adjustments  made  according  to  individual  taste.  Through 
lack  of  accurate  methods  of  test  and  of  conclusive  evidence 
on  the  subject,  it  was  quite  common  to  find  extremists  who, 
on  the  one  hand,  denied  the  efficacy  of  suspensions  in  general, 
and  on  the  other  ardently  supported  crazily  conceived  sup- 
porting arrangements  which  accurate  comparative  test  show 
to  be  even  worse  than  useless. 

In  the  French  service,  despite  numerous  types  of  sus- 
pension available,  the  very  general  practice  was  to  lift 
the  camera  from  its  support  and  hold  it  between  the  knees. 
Or  else  the  hand  was  pressed  on  the  top  of  the  camera 
during  exposure,  more  reliance  being  placed  on  the  damp- 
ing qualities  of  the  body  than  on  any  of  the  rubber  or 
spring  mechanisms. 

As  is  clearly  shown  by  the  experimental  data  described 
in  the  last  chapter,  a  correctly  designed  supporting  device, 
carrying  the  camera  accurately  in  the  plane  of  its  center  of 
gravity,  accomplishes  practically  perfect  elimination  of 
vibrational  troubles.  So  important  is  the  use  of  a  mount  and 
so  important  is  it  that  the  mount  should  be  correctly  dimen- 
sioned and  adjusted  for  the  camera,  that  an  entirely  different 
attitude  should  be  adopted  from  the  prevalent  one  which 
focuses  attention  on  the  camera  and  regards  the  mounting 

13  193 


194        AIRPLANE  PHOTOGRAPHY 


as  a  mere  auxiliary  to  be  left  more  or  less  to  chance.  The 
mounting  should  be  considered  an  integral  part  of  the  camera. 
The  man  in  the  field  should  receive  camera  and  mount 
together,  leaving  as  his  only  problem  the  attachment  of  the 
complete  camera — and — mount  unit  to  the  plane.  This  may 
be  arranged,  by  proper  designing,  to  be  a  simple  matter 


V 


FIG.  81. — "L"  camera   mounted  outside  the  fuselage.    Observer  using  exposure  plunger,  pilot  using 

Bowden  wire  release. 

of  rigid  bolting  or  strapping,  requiring  ingenuity  perhaps 
but  not  the  scientific  knowledge  which  is  required  for 
mounting  design. 

Outboard  Mountings. — In  the  English  service  the 
camera  was  first  attached  to  the  plane  outside  the  fuselage 
by  a  rigid  frame,  to  which  the  camera  was  strapped  or 


CAMERA  MOUNTINGS  195 

bolted  (Fig.  81).  Obvious  objections  exist  to  placing  the 
camera  in  this  position,  such  as  the  resistance  of  the  wind 
and  the  difficulty  of  changing  magazines.  However,  in  the 
earlier  English  planes  with  their  fuselages  of  small  cross 
section  no  other  accessible  place  for  the  camera  was  to  be 
found.  Vibrational  disturbances  with  the  rigid  outboard 
mounting  are  quite  serious,  as  is  so  clearly  indicated  by  the 
trace  shown  in  Fig.  78.  Extremely  short  exposures  are 
alone  possible,  and  a  very  large  proportion  of  the  pictures 
are  apt  to  be  indistinct. 

Floor  Mountings. — A  step  in  advance  of  the  outboard 
mounting  is  to  support  the  camera  snout  in  a  padded  conical 
frame  on  the  floor  of  the  plane  (Fig.  82).  This  mounting 
avoids  the  objection  on  the  ground  of  wind  resistance  that 
holds  with  the  outboard,  and  has  possibilities  of  being  worked 
out  as  an  entirely  satisfactory  support.  Yet  to  be  satis- 
factory, the  point  of  support  must  lie  in  the  plane  of  the 
center  of  gravity  of  the  camera,  and  the  camera  must  be  of  a 
type  that  preserves  its  center  of  gravity  unchanged  in  posi- 
tion as  the  plates  are  exposed.  Unless  these  conditions  are 
fully  met  the  floor  mounting  gives  results  little  better  than 
does  the  outboard. 

Cradles  or  Trays. — Floor  space  in  the  cockpit  being 
unavailable  in  the  battle-plane,  due  to  duplicate  controls, 
bomb  sights,  etc.,  the  English  service  was  driven  to  the  prac- 
tice of  carrying  the  camera  in  the  compartment  or  bay 
behind  the  observer.  Here  it  was  attached  either  to  the 
structural  uprights  or  longerons,  or  to  special  uprights  and 
cross-pieces  built  into  the  plane  to  serve  photographic  ends. 
As  an  intermediary  between  the  camera  and  the  supporting 
cross-pieces  there  was  introduced  the  camera  tray  or  cradle. 
This  is  essentially  a  frame  carrying  sponge  rubber  pads  into 
which  the  camera  is  more  or  less  deeply  bedded.  Figs.  83  and 


196 


AIRPLANE  PHOTOGRAPHY 


84  show  an  American  L  camera  cradle  based  on  the  design 
of  the  English  L  camera  tray.  Thick  sponge  rubber  pads 
support  the  two  ends  of  the  camera  top  plate,  and  additional 
pads  are  provided  to  hold  the  nose  of  the  camera.  Careful 


FIG.  82. — "L"  camera  in  floor  mounting. 

tests  show  this  cradle  to  be  superior  to  the  outboard  mount- 
ing, but  still  leave  much  to  be  desired.  Its  performance  is 
better  with  the  nose  of  the  camera  left  free. 

Tennis=ball  Mounting. — A  very  simple  mount  used  by 
the  French  consists  of  a  frame  enclosing  the  nose  of  the 


CAMERA  MOUNTINGS 


197 


camera,  and  carrying  four  tennis  balls,  on  which  the  whole 
weight  rests  (Fig.  40).  If  the  center  of  support  is  in  the  plane 
of  the  center  of  gravity  and  if  the  four  balls  are  of  uniform 


FIG.  83. — "L"  camera  and  cradle  mount  in  skeleton  DeHaviland  4  fuselage,  side  view. 

age  and  elasticity,  this  form  of  support  is  good.  As  provided 
by  the  camera  manufacturer,  the  tennis  ball  frame  fits  much 
too  far  down  on  the  camera.  Another  application  of  the 


198 


AIRPLANE  PHOTOGRAPHY 


tennis  ball  idea  was  frequently  made  in  the  French  service, 
in  which  the  balls  were  close  up  under  the  shutter  housing 
(Fig.  85).  Additional  support  was,  however,  given  to  the 
camera  nose  by  flexible  rubber  bands,  the  success  of  the 
whole  being  largely  a  matter  of  the  adjustment  of  the  tension 
on  the  bands. 


FIG.  84. — "L"  camen 


T  :\   J 
!e  mountin  skeleton  deHaviland  4  fuselage,  front  view. 


Parallel  Motion  Devices. — A  form  of  suspension  favored 
by  the  French  consists  of  parallel  bell  cranks,  rigidly  linked 
together  and  held  up  by  springs.  Mountings  of  this  sort 
are  illustrated  in  Figs.  86,  87,  88  and  96.  The  guiding  prin- 
ciple is  that  any  sort  of  shock  will  be  transformed  into  a 
straight  up-and-down  or  side-wise  motion  of  the  camera, 
which  is  harmless.  The  mounting  as  adapted  by  the  English 
surrounds  the  camera  body,  making  the  plane  of  support 


CAMERAMOUN  TINGS  ' 


199 


somewhere  near  the  center  of  gravity.  In  certain  of  the 
French  suspensions  employing  this  principle  the  whole 
camera  is  hung  below  the  bell  cranks  (Fig.  86),  and  then  the 


FIG.  85. — Tennis  ball  suspension,  assisted  by  elastic  bands  attached  to  nose  of  camera,, 

nose  is  restrained  by  heavy  rubber  bands.     The  net  result 
is  largely  a  matter  of  adjustment. 

Tests  on  the  English  design  made  in  the  United  States 


200        AIRPLANE  PHOTOGRAPHY 


FIG.  86. — French  spring  and  bell  crank  suspensic 


CAMERA  MOUNTINGS' 


201 


202        AIRPLANE  PHOTOGRAPHY 


FIG.  89. — Testa  of  camera  mountings:  (a)  deRam  camera  on  bell-crank-and-spring  mount, 
below  the  center  of  gravity;  (6)  same,  at  center  of  gravity;  (c)  type  "K"  film  camera  on  universal 
mounting  (Fig.  88). 


CAMERA  MOUNT  INGS'  203 

Air  Service  appear  to  show  that  the  chief  virtue  of  the  mount- 
ing lies  in  the  approximation  of  the  point  of  support  to  the 
center  of  gravity  in  the  English  cameras.  A  deRam  camera 
supported  by  its  cone,  so  that  its  center  of  gravity  was  con- 
siderably above  the  center  of  support  gave  rather  poor  results 
(Fig.  89a),  but  when  the  bell  cranks  were  attached  near  the 
center  of  gravity,  highly  successful  results  were  obtained 
(Fig.  896).  The  French  deRam  camera  as  ordered  for  the 
American  Expeditionary  Force  was  fitted  with  a  bell  crank 
supported  in  this  position. 

Figures  90  and  91  show  a  bell  crank  mounting  furnished 
with  a  rotating  turret.  This  was  designed  to  facilitate  the 
changing  of  magazines  in  the  English  B  M  camera,  which 
is  swung  around  through  90  degrees  from  the  exposing 
position  to  bring  the  "magazine  near  the  observer.  The 
camera  shown  in  the  mounting  is  the  American  hand-operated 
model  (type  M),  in.  which  there  is  the  same  necessity  for 
turning  in  order  to  manipulate  the  bag  magazine  easily. 
The  camera  is  shown  in  both  exposing  and  plate  changing 
positions.  An  important  detail  of  these  mounts  is  a  safety 
catch,  which  must  be  fastened  before  the  plane  lands,  in 
order  to  prevent  the  shocks  of  landing  from  producing 
oscillations  sufficient  to  throw  the  camera  out  of  the  mount. 

Center  of  Gravity  Rubber  Pad  Supports. — Given  a  camera 
whose  center  of  gravity  does  not  change  during  operation,  a 
simple  and  entirely  adequate  anti- vibration  support  is  fur- 
nished by  a  ring  of  sponge  rubber  in  the  plane  of  the  center  of 
gravity.  But  if  provision  has  to  be  made  for  oblique  views 
or  for  adjusting  the  camera  to  the  vertical,  something  more 
elaborate  is  necessary. 

Mountings  for  the  American  deRam  and  for  the  Air 
Service  film  camera,  embodying  the  results  of  complete 
study  of  the  anti- vibration  problem,  are  shown  in  Figs.  90, 


204        AIRPLANE  PHOTOGRAPHY 


CAMERA  MOUNTINGS 


205 


92  and  93.    Trusses  carrying  the  cameras  on  pivots  rest  on 
four  pads  of  sponge  rubber  which  are  mounted  on  frames 


FIG.  92. — U.   S.   type   "K"   film   camera   on    universal  mounting,  vertical  position. 


FIG.  93. — U.  S.  type  "K"  film  camera  on  universal  mounting,  oblique  position. 

correctly  spaced  ready  for  attachment  to  the  cross-pieces 
of  the  airplane  camera  supports.  In  the  deRam  (Fig.  90) 
the  pivots,  attached  to  the  camera  body,  permit  it  to  be 


206 


AIRPLANE  PHOTOGRAPHY 


leveled  fore  and  aft,  to  compensate  for  the  inclined  position 
of  the  fuselage  assumed  at  high  altitudes  or  in  some  condi- 


I    S  ec 


J-N  -4 ,ri* 


FIG.  94.— Tests  on  two  types  of  camera  mount:      (a)  Support  at  bottom  of  camera;     (6)  support 

above  center  of  gravity. 

tions  of  loading.  This  will  sometimes  amount  to  as  much 
as  11  or  12  degrees,  which  is  very  serious,  since  one  degree 
causes  (with  an  angular  field  of  20  degrees)  about  one  per 


CAMERA  MOUNTINGS  207 

cent,  difference  of  scale  at  the  two  sides  of  the  plate.  The 
film  camera  mounting  carries  the  camera  in  a  conical  ring, 
and  is  pivoted  not  only  for  vertical  adjustment,  but  for  the 
taking  of  obliques  as  well  (Fig.  93).  These  mounts  transmit 
practically  no  vibration. 

A  caution  must  be  noted  with  regard  to  center  of  gravity 
mountings.  Any  change  in  the  camera,  in  particular  the 
substitution  of  a  short  for  a  long  lens  cone,  must  be  made  so 
as  to  cause  no  alteration  of  the  relative  positions  of  the  center 
of  support  and  the  center  of  gravity.  Either  the  short  cone 
must  be  weighted,  or  additional  supporting  pivots  must  be 
provided  in  the  plane  of  the  new  center  of  gravity. 

The  Italian  and  G.  E.  M.  Mountings. — These  mounts 
(Figs.  49  and  59)  are  similar  in  that  the  protection  from 
vibration  is  furnished  by  an  elastic  support  at  the  bottom  of 
the  camera.  Tests  show  that  these  two  cameras  give  very 
similar  results,  of  the  unsatisfactory  sort  to  be  expected  from 
this  kind  of  mounting  in  view  of  the  lessons  of  the  last 
chapter  on  the  proper  point  of  support.  Fig.  94,  a,  shows  a 
trace  given  by  the  Italian  mount.  The  permissible  exposure, 
on  the  criterion  adopted,  is  very  short  with  either  mount, 
about  -2^-0  second. 

The  Brock  Suspension. — This  consists  of  a  pair  of  frames 
into  which  the  camera  is  fitted  by  ball  bearing  pivots,  so 
that  it  is  free  to  move  in  any  direction  (Fig.  60).  In  order  to 
permit  gravity  to  control  the  direction  of  the  camera,  the 
point  of  support  is  made  considerably  (ten  inches)  above  the 
center  of  gravity.  Air  dash  pots  are  provided  for  damping  the 
swings.  As  already  explained,  the  pendular  method  of  sup- 
port is  in  basic  contradiction  to  the  requirements  for  vibra- 
tion elimination.  Tests  of  the  Brock  suspension, shownin  Fig. 
94,  b,  indicate  it  to  be  of  low  efficiency  in  damping  out  the  short 
period  vibrations  which  are  responsible  for  poor  definition. 


CHAPTER  XVI 

THE  INSTALLATION  OF  CAMERAS  AND  MOUNT- 
INGS IN  PLANES 

Conditions  to  Be  Met. — The  characteristic  difficulty 
in  installing  the  airplane  camera  is  that  there  is  no  place  for 
it.  After  the  gasoline  supply,  the  armament,  the  wireless, 
the  oxygen  tank,  the  bombs,  and  other  necessities  are  taken 
care  of  there  is  neither  space  available  nor  weight  allowable. 
Where  space  may  be  found  it  will  be  inaccessible,  or  acces- 
sible only  through  a  maze  of  tension  and  control  wire  3;  or 
it  will  be  in  a  position  where  any  weight  will  endanger  the 
balance  of  the  plane.  Plane  design  has  in  fact  been  more  or 
less  of  a  conflict  between  the  aeronautical  engineer,  who  is 
designing  the  airplane  primarily  as  a  machine  to  fly,  and  the 
armament  and  instrument  men,  who  look  upon  it  as  a  plat- 
form for  their  apparatus.  Lack  of  appreciation  of  the 
extreme  importance  of  aerial  photography  resulted,  during  a 
large  part  of  the  war,  in  the  camera  installation  being  neg- 
lected until  the  plane  was  supposedly  entirely  designed,  and 
even  in  production.  At  that  stage  the  installation  could  be 
but  a  makeshift.  Only  in  the  later  stages  of  the  war,  when 
plane  design  became  a  matter  of  cooperation  between  all 
concerned,  were  fairly  convenient  and  satisfactory  arrange- 
ments made  for  the  camera.  Always,  however,  the  rapid  suc- 
cession of  new  plane  designs,  with  various  shapes  of  fuselage 
and  details  of  structure,  made  camera  installation  in  the 
war  plane  a  matter  calling  for  the  greatest  ingenuity. 

The  problem  was  met  in  part  by  constructing  both 
cameras  and  mountings  in  sections,  to  be  laboriously  wormed 
in  through  inadequate  apertures,  in  part  by  later  structural 

208 


CAMERAS  AND  MOUNTINGS      £09 

changes  in  the  planes,  such  as  the  substitution  of  veneer 
rings  or  frames  for  the  tension  wires.  In  certain  cases  the 
rear  cockpit  controls  were  omitted,  thereby  freeing  accessible 
and  often  adequate  space  for  the  larger  cameras.  Rear  con- 
trols were  never  used  in  the  German  planes,  so  that  their 
standard  practice  was  to  carry  the  camera  forward  of  the 
observer.  This,  together  with  the  general  restriction  to  the 
13X18  centimeter  size  plate,  made  the  installation  problem 
less  difficult  in  the  German  aircraft  than  in  the  Allied. 

Practical  Solutions. — An  important  feature  of  camera 
installation  has  already  been  mentioned,  but  may  well  be 
repeated  for  emphasis.  The  camera  and  its  anti- vibration 
mounting  should  always  be  considered  as  a  unit,  and  should 
be  so  designed  that  simple  bolts  or  straps  will  suffice  to  fasten 
it  in  its  place  in  the  plane.  Even  should  the  spacing  of  the 
structural  parts  of  the  plane  not  correspond  to  that  antici- 
pated by  the  mounting  design,  the  ingenuity  of  the  man  in 
the  field  may  be  depended  upon  to  make  the  necessary 
alterations  or  additions  to  the  plane.  The  design  of  the 
camera  suspension  itself  cannot,  however,  be  left  to  unedu- 
cated ingenuity. 

Assuming  the  camera  and  mounting  supplied,  the  next 
step — a  very  difficult  one — is  to  insure  uniformity  in  the 
structures  to  be  built  into  the  planes  for  the  purpose  of  sup- 
porting the  camera  mountings.  With  this  uniformity  must, 
however,  be  combined  the  greatest  possible  flexibility  to 
provide  for  various  designs  of  cameras. 

In  the  English  service  the  standard  camera  installation 
consists  of  wooden  uprights  with  cross  bars  athwart  the 
plane,  adjustable  as  to  height  (Fig.  95).  A  distance  between 
the  cross  bars  of  13J4  inches  has  been  standardized,  and  all 
camera  cradles  and  mountings  are  notched  or  otherwise 
spaced  to  fit  this  dimension.  The  installation  adopted  in 
14 


210 


AIRPLANE  PHOTOGRAPHY 


the  American  planes  is  similar,  but  with  a  distance  of  16 
inches  between  cross  bars.  These  uprights  and  cross  bars 
are  ordinarily  situated  in  the  bay  behind  the  observer,  but 
can  be  placed  in  any  available  space.  Fig.  83  shows,  in  a 
model  bay,  the  arrangement  of  uprights  and  cross  bars  in 


FIG.  95. — "L-B"  camera  with  20-inch  lens,  mounted  on  bell-crank  suspension  in  skeleton 
fuselage.  Stream-lined  hood  below  to  cover  projecting  end  of  lens  cylinder.  Propeller  and  Bowden 
release  in  place. 

the  American  DH  4,  with  the  L  camera  in  place  in  its  cradle. 
It  is  just  possible  to  introduce  camera  and  cradle  separately 
from  the  observer's  cockpit  through  the  tension  wires,  and, 
by  uncomfortable  reaching,  magazines  may  be  changed. 

A  step  in  advance  is  made  when  the  top  tension  wires 
and  superstructure  are  replaced  by  a  rigid  frame  with  an 


CAMERAS  AND   MOUNTINGS 

opening  large  enough  to  admit  the  entire  camera  and  mount- 
ing. When  this  is  done  considerably  larger  cameras  may  be 
accommodated  in  the  same  sized  bay,  as  shown  in  Fig.  96. 
A  further  advance,  from  the  standpoint  of  accessibility  and 
convenience  of  installation,  follows  when  the  tension  wires 
between  observer's  and  camera  bay  are  replaced  by  a  ply- 
wood ring,  as  shown  in  Fig.  97.  Here  the  only  serious  limi- 
tations are  those  due  to  the  vertical  height  of  the  camera, 
and  of  course  its  weight. 

Openings  for  the  lens  to  point  through  are  simply  provided 
in  the  fabric  covered  aircraft,  by  cutting  through  the  canvas 
and  stiffening  the  edge  of  the  hole  by  wire.  Tension  wires 
are  often  in  the  way.  They  may  either  be  disregarded,  since 
they  merely  cut  off  a  little  light,  or  replaced  in  part  by  metal 
rings,  as  shown  in  Fig.  96.  In  veneer  covered  fuselages  the 
hole  must  of  course  go  through  the  wood.  This  may  be 
undesirable,  since  the  veneer  is  depended  on  to  furnish 
structural  strength,  a  point  which  further  emphasizes  the 
importance  of  the  photographic  requirements  being  thor- 
oughly considered  while  the  plane  is  being  designed. 

Single  seater  or  scout  planes  do  not  lend  themselves  to 
the  insertion  of  such  standardized  uprights  and  cross-pieces, 
because  of  their  small  size  and  the  common  utilization  of  all 
space  inside  the  fuselage  for  gasoline  tanks  and  control 
wires.  Some  French  scouts,  whose  fuselages  are  very  wide, 
due  to  the  rotary  engines,  have  been  fitted  with  compart- 
ments for  contemplated  automatic  film  cameras.  The  most 
commonly  used  camera  in  the  single  seater  was,  however,  the 
Italian  24-plate  single-motion  apparatus  (Fig.  49).  This 
camera  and  its  carrying  tray  occupy  very  little  lateral  space 
and  have  in  actual  practice  been  carried  beneath  the  seat 
or  pushed  up  through  an  opening  in  the  bottom  of  the  fusel- 
age under  the  gasoline  tank.  Whatever  criticism  may  be 


AIRPLANE  PHOTOGRAPHY 


ll 

a 


If 


•  t^     U 

£3*8-5 

ill 


CAMERAS  AND  MOUNTINGS      213 

made  of  the  adequacy  of  the  mounting,  it  must  be  said  that 
the  camera,  as  used,  is  perhaps  the  most  eminently  practical 
of  all  developed  in  the  war,  as  its  use  on  scouts  testifies. 

Special  Photographic  Planes. — As  cameras  grew  in  size, 
the  difficulty  of  installing  them  in  planes  built  without  regard 
to  photographic  requirements  greatly  increased.  Few  planes 
could  carry  even  the  50  centimeter  focus  camera  obliquely 
without  the  necessity  of  poking  its  nose  through  the  side 
where  it  would  catch  wind  and  oil;  while  the  120  centimeter 
camera  could  be  carried  obliquely  only  in  the  fore  and  aft 
position.  Even  vertical  installation  of  the  latter  camera  was 
really  feasible  in  but  few  planes;  sometimes  the  camera  was 
carried  to  the  exclusion  of  the  observer — and,  in  fact,  this 
size  was  never  used  by  the  English,  whose  fuselages  were 
small  in  cross-section. 

This  situation  led,  late  in  the  war,  to  steps  toward  pro- 
ducing planes  designed  primarily  for  photographic  recon- 
naissance. In  these  the  camera  would  be  entirely  accessible, 
and  cameras  of  any  size  could  be  carried  in  any  desired 
position.  One  scheme  which  properly  belongs  under  this 
heading  was  the  provision  of  a  special  removable  photo- 
graphic cockpit,  for  the  front  or  nose  of  a  twin-motored  three 
seater.  Other  noses,  for  bombing  and  heavy  machine  guns, 
were  also  planned,  all  to  be  interchangeable.  Since  the 
regular  photographic  bay  with  uprights  and  cross-pieces  was 
also  provided  to  the  rear,  this  special  photographic  ship 
could  on  occasion  do  two  classes  of  work,  such  as  long  focus 
spotting  and  short  focus  mapping. 

The  most  completely  worked  out  photographic  plane  was 
probably  the  model  designated  Pi  by  the  United  States  Air 
Service.  This  is  a  modified  de  Haviland  4  in  which  the  rear 
controls  have  been  removed  and  the  cowling  raised  and  at  the 
same  time  made  squarer  in  cross-section.  The  space  formerly 


£14        AIRPLANE  PHOTOGRAPHY 

occupied  by  the  rear  controls  provides  ample  room  for  all 
types  of  camera.  These  are  carried  on  uprights  at  the 
standard  distance  apart,  16  inches,  with  cross-pieces  adjust- 
able as  to  height.  The  camera  space  is  accessible  not  only 
from  the  observer's  cockpit,  but  from  above,  upon  folding 
back  the  metal  cover.  Doors  at  the  bottom  and  at  each  side 
permit  not  only  vertical  but  oblique  exposures.  The  latter 
are  not  interfered  with  by  the  wings,  as  they  would  be  in 
some  designs  of  plane  if  the  camera  occupied  the  same  posi- 
tion relative  to  the  cockpits.  Fig.  91  shows  the  deRam 
camera  in  place,  as  seen  from  the  rear.  Figs.  98  and  99  show 
the  18X24  centimeter  film  camera,  set  both  for  vertical  and 
oblique  views. 

Negative  lenses  are  provided  for  both  pilot  and  observer, 
the  one  for  the  pilot  permitting  him  to  see  from  a  point  far 
ahead  to  directly  underneath,  while  the  observer's  is  fur- 
nished with  cross  wires  below  and  etched  rectangles  of  the 
camera  field  sizes  on  the  upper  surface.  Windows  of  non- 
breakable  glass  assist  in  sighting  obliques.  The  accompany- 
ing picture  (Fig.  100)  of  the  plane  showing  an  oblique  camera 
in  position  gives  an  excellent  idea  of  its  appearance.  Its 
special  features  are  worthy  of  copying  in  peace-time  photo- 
graphic aircraft. 

Installation  of  Auxiliaries. — It  is  quite  necessary  that 
the  camera  lens  be  protected  from  splashing  mud  and  often 
from  oil  spray  due  to  the  motor.  For  this  purpose  an  easily 
opened  and  closed  door  is  essential,  unless  the  camera  is 
carried  well  up  in  the  plane.  An  alternative,  possessing 
certain  advantages,  is  to  incorporate  into  the  camera  pro- 
tecting flaps  operating  in  front  of  the  lens,  which  open  only 
when  the  exposure  is  made.  If  the  camera  projects  beyond 
the  fuselage,  stream  lined  hoods  (Fig.  95)  must  be  provided  to 
protect  the  camera  nose  with  the  minimum  of  air  resistance. 


CAMERAS  AND   MOUNTINGS       215 


216        AIRPLANE  PHOTOGRAPHY 


CAMERAS  AND  MOUNTINGS       217 

The  mounting  of  the  regular  camera  auxiliaries — releases, 
sights,  propellers,  speed  controls,  motors — is  usually  a  great 
bother,  due  to  lack  of  space  and  to  the  severe  restrictions  on 
methods  of  fastening.  Screws  in  longerons  or  uprights  are 
taboo.  Metal  straps  to  go  around  structural  parts  are  the 
approved  device,  but  with  variations  in  the  size  of  these 
members,  the  holes,  straps,  bolts  and  nuts  provided  are  very 
apt  not  to  fit.  Changes  of  construction,  such  as  that  from 
skeletons  covered  with  fabric  to  veneer  bodies,  also  interfere 
with  any  standard  means  of  attachment,  and  leave  this,  like 
many  other  problems  in  war-time  aerial  photography,  to 
the  resourcefulness  of  the  man  in  the  field. 

Magazine  racks  must  be  tucked  away  in  any  available 
space.  Under  the  seat  is  a  position  frequently  utilized. 
Especially  with  plates  is  it  desirable  to  carry  the  extra 
magazines  in  a  position  to  interfere  as  little  as  possible  with 
the  balance  of  the  plane.  In  the  DH  4  this  means  that  they 
should  be  carried  if  possible  forward  of  the  observer,  even 
though  he  must  turn  completely  around  to  get  and  insert 
each  magazine. 


IV 

SENSITIZED  MATERIALS  AND  CHEMICALS 


CHAPTER  XVII 

THE    DISTRIBUTION    OF    LIGHT,    SHADE    AND 
COLOR  IN  THE  AERIAL  VIEW 

The  general  appearance  of  the  earth  as  viewed  from  above 
has  already  been  described  and  illustrated  (Figs.  10  and  11). 
It  remains  to  deal  with  the  earth's  appearance  in  a  more 
analytic  and  quantitative  manner,  in  order  to  decide  upon 
the  characteristics  to  be  sought  in  our  photographic  sensi- 
tive materials. 

Range  of  Brightness. — The  absence  of  great  contrasts 
so  apparent  in  the  view  of  the  earth  from  a  plane  is  confirmed 
by  photometric  observations.  These  show  that  the  average 
landscape,  as  seen  from  the  air,  rarely  presents  a  range  of 
brightness  of  more  than  seven  to  one,  even  when  seen  with- 
out the  presence  of  veiling  haze.  It  is  to  be  remembered  that 
shadows  constitute  no  important  part  of  the  aerial  landscape. 
Vertical  walls  in  shadow,  which  form  a  substantial  part  of 
the  surfaces  seen  by  an  observer  on  the  ground,  are  invisible 
or  greatly  foreshortened  from  the  air.  Moreover,  they  are 
never  contrasted  against  the  sky,  which  is  photographically 
often  the  brightest  part  of  the  ordinary  picture.  To  the 
aviator's  eye  shadows  on  the  ground  are  only  of  any  length 
at  early  and  late  daylight  hours.  Even  at  these  times  they 
cover  but  a  small  area,  since  the  number  of  high  vertically 
projecting  objects  in  a  representative  landscape  is  small. 
Lacking  shadows,  the  brightness  range  is  only  that  between 
various  kinds  of  earth,  water,  and  vegetation.  Chalk  (from 
freshly  dug  trenches),  reflected  sunlight  from  water,  or 
marble  buildings,  furnish  almost  the  only  extensions  to  the 
brightness  scale  as  above  given. 

221 


222 


AIRPLANE  PHOTOGRAPHY 


Diurnal  and  seasonal  changes.  During  the  winter 
months  on  the  Western  Front  photography  from  the  air 
was  only  possible  for  two  or  three  hours  around  noon,  on 
clear  days.  This  calls  attention  to  another  factor  of  prime 
importance,  namely,  the  large  variation  in  the  intensity  of 


1 

f 

40 
20 

/ 

5 

/ 

V 

/ 

\ 

7 

*. 

\ 

\ 

y 

' 

/ 

\ 

\ 

/ 

I 

\ 
\ 

\ 

/ 

/ 

\ 
\ 

/ 

i 

\ 

\ 

j 

/ 
/ 

\ 

V 

/ 

\ 

4-A.fl  8  H  4  StfAl 

FIG.  101. — Variation  of  average  daylight  intensity  during  the  day. 

daylight  during  the  course  of  the  day  and  during  the  course 
of  the  year. 

Measurements  showing  typical  variations  from  morning 
to  night  are  exhibited  in  Fig.  101,  from  which  it  appears 
that  there  is  an  increase  in  illumination  of  four  to  five  times 
from  8  o'clock — when  it  would  be  considered  full  daylight 
for  purely  visual  observation — until  noon,  while  there  is  a 
corresponding  decrease  by  four  o'clock.  Fig.  102  shows  sets 


LIGHT,  SHADE  AND  COLO'R 


of  measurements  by  two  different  authorities  which  give 
the  average  intensity  of  daylight  for  each  month  throughout 
the  year.  From  December  to  July  there  is  an  increase  of 
approximately  ten  times.  From  both  sets  of  data  it  there- 
fore appears  that — neglecting  the  frequent  occurrence  of 
clouds  which  reduce  the  illumination  to  a  half  or  a  quarter 
or  even  less — a  variation  in  illumination  of  forty  or  fifty 


Intensity  of  daylighr 

/ 

/ 

""""I 
/ 

\ 

\ 
\ 

80 
loO 

4-0 
20 

/ 

/ 

/ 

\ 
\ 

\ 
\ 

/ 
/ 

/ 
/ 

\ 

V 
\ 

/ 
1 

/ 
/ 

\ 

X 

^ 

—  Vi 

\ 

/ 

/ 

\ 

\\ 

/ 

f 
/ 

'^•5*' 

\  \ 

\\ 

/ 

1 

/ 
/ 

\ 

\ 

\ 

/ 

/ 
/ 

\ 

,    \ 

X 

/ 

\ 

feb  Apr  June  Aog  Oct  Pec. 

FIG.  102. — Variation  of  intensity  of  daylight  through  the  year;  two  different  sets  of  measurements. 

times  occurs  between  mid-day  in  summer  and  morning  in 
winter.  In  the  photography  of  stationary  objects  on  the 
ground  this  range  of  intensities  is  easily  taken  care  of  by 
selection  of  lens  stop  and  shutter  speed.  On  the  airplane 
it  is  quite  otherwise,  because  the  shutter  speeds  called  for 
at  the  lower  illuminations  are  much  slower  than  the  motion 
of  the  plane  will  allow. 

Haze. — At  low  altitudes  the  brightness  range  is  sub- 
stantially that  which  would  be  obtained  by  photometric 


224        AIRPLANE  PHOTOGRAPHY 

measurements  of  soil  and  vegetation  made  at  the  earth's 
surface.  At  higher  altitudes,  especially  above  2000  meters, 
this  brightness  range  is  materially  decreased  by  atmospheric 
haze.  The  significance  of  this  lies  in  the  fact  that  for  safety 
from  anti-aircraft  guns,  war-time  aerial  photography  must 
be  carried  out  at  very  great  elevations.  Toward  the  end  of 
the  Great  War  photographic  missions  traveling  at  from  5000 
to  7000  meters  were  the  rule.  At  these  heights,  even  in  very 
clear  weather,  a  veil  of  bluish- white  haze  reduces  the  already 
small  contrasts  still  more.  Some  means  for  overcoming  the 
effect  of  this  haze  becomes  imperative,  therefore,  in  order 
to  secure  in  the  picture  even  the  normal  contrast  of  the  object. 

Haze  is  to  be  sharply  distinguished  from  clouds  or  fog. 
Clouds  and  fog  consist  of  globules  of  water  vapor  of  large 
size,  opaque  to  light.  Haze,  on  the  contrary ,  is  more  opaque 
to  some  colors  than  to  others,  or  is  selective  in  its  veiling 
effect.  Its  scattering  action  on  light  is  greatest  in  the  violet 
and  blue  of  the  spectrum,  decreasing  rapidly  through  the 
green,  yellow,  and  red,  the  exact  relation  being  that  the 
scattering  is  inversely  as  the  fourth  power  of  the  wave- 
length. It  is,  consequently,  possible  to  pierce  or  cut  haze  by 
using  yellow,  orange,  or  red  color  screens.  It  is  this  possi- 
bility which  has  led  to  the  extensive  use  of  yellow  or  orange 
goggles  for  shooting  and  for  naval  lookout  work.  In  aerial 
photography  the  equivalent  is  to  be  found  in  color  filters, 
used  with  color  sensitive  (orthochromatic  or  panchromatic) 
plates,  which  have  been  found  essential  for  all  high  alti- 
tude work. 

Color. — Visual  observation  from  the  airplane  is  aided  in 
no  inconsiderable  degree  by  the  differences  of  color  that  exist 
between  various  objects  of  nearly  the  same  brightness.  This 
means  of  distinguishing  differences  of  character  fails  in  the 
photographic  plate,  which  is  color-blind;  that  is,  it  reproduces 


LIGHT,  SHADE  AND  COLOR 

all  objects  as  grays  of  varying  brightness.  It  is  color-blind 
in  another  sense  as  well,  in  that  it  evaluates  colors  as  to 
brightness  differently  from  the  way  the  eye  does,  overrating 
blues  and  violets  and  underrating  yellows  and  reds.  This 
first  kind  of  color-blindness  is  a  positive  disadvantage,  for  it 
leaves  available  for  differentiating  objects  only  their  bright- 
ness differences.  The  second  kind  of  color-blindness  may  on 
occasion  actually  be  an  advantage.  For  it  may  happenx  by 
accident,  or  by  design  (through  the  skilful  use  of  color  filters), 
that  objects  appearing  nearly  the  same  to  the  eye  appear 
different  in  the  plate.  More  will  be  said  about  this  in  con- 
nection with  the  use  of  filters  for  the  detection  of  camouflage. 

The  range  of  hues  seen  in  the  aerial  landscape  is  not 
large.  Greens  (grass  and  foliage)  predominate,  followed  by 
browns  (earth),  neither  color  being  bright  or  saturated.  Over 
towns  or  cities  we  find  that  grays  (roads)  and  redder  browns 
(brick)  are  conspicuous.  Blues  are  practically  never  seen, 
although  it  is  to  be  noted  that  a  fair  share  of  the  illumination 
of  the  ground  is  by  blue  sky  light  and  that  the  haze  itself  is 
bluish.  Consequently,  the  general  tone  of  a  landscape  is 
much  bluer  than  one  would  be  apt  to  imagine  it  from  con- 
sideration of  the  general  green  and  brown  character  of  the 
constituent  objects.  A  color  photograph  from  the  air  would 
greatly  resemble  a  pastel  in  its  low  range  of  tones  and  the 
absence  of  bright  colors. 

The  Photographic  Requirements  Dictated  by  Brightness 
and  Color  Considerations. — Considering  only  the  demands 
made  by  the  character  of  the  view  presented  to  the  airplane 
camera,  and  leaving  out  of  account  other  limitations  to 
photographic  operations  in  the  plane,  certain  requirements 
as  to  sensitized  materials  may  be  outlined.  First  of  all,  the 
photographic  process  must  not  reduce,  but  should  rather  be 
capable  of  exaggerating,  the  range  of  brightness  of  the  object. 
15 


AIRPLANE  PHOTOGRAPHY 

Preferably  the  seven-to-one  range  of  the  object  photographed 
should  be  lengthened  out  to  the  full  range  of  the  printing 
paper,  which  may  be  two  to  three  times  this.  With  such  an 
increase  of  range,  those  minute  differences  of  brightness  are 
accentuated,  on  which  the  detection  of  many  objects  depends. 

Next,  the  plate  or  film  must  be  sensitive  to  the  portion  of 
the  spectrum  transmitted  by  a  yellow  or  orange  filter  which 
will  cut  out  the  effect  of  haze.  This  calls  for  orthochromatic 
or  panchromatic  plates,  depending  on  the  depth  of  filter 
required.  Next,  if  the  objects  to  be  photographed  differ 
little  in  brightness  but  are  different  in  color  composition, 
we  may  have  to  rely  on  color  filters  of  peculiar  transmissions, 
capable  of  translating  these  color  differences  into  brightness 
differences.  These  will,  in  general,  call  for  fully  color  sensi- 
tive, or  panchromatic  plates. 

In  conclusion  it  may  be  pointed  out  that  the  endeavor  in 
ordinary  orthochromatic  photography — to  reproduce  the 
visual  brightness  of  colors  in  the  photographic  print — has  no 
real  justification  in  aerial  work.  Neither  in  respect  to  color 
values  nor  in  respect  to  brightness  range  is  it  the  object  of 
aerial  photography,  especially  for  war  purposes,  to  present  a 
truthful  tone  reproduction.  Its  aim  is  rather  the  adequate 
differentiation  of  detail,  by  whatever  means  necessary. 


CHAPTER  XVIII 

CHARACTERISTICS  OF  PHOTOGRAPHIC 
EMULSIONS 

The  purely  photographic  problem  in  aerial  photography, 
as  distinct  from  the  instrumental  one,  is  the  selection  of 
photo  sensitive  materials  which  will  yield  useful  results  under 
the  conditions  peculiar  to  exposure  from  the  air.  After  such 
materials  have  been  found  by  extensive  field  tests,  it  is  pre- 
eminently desirable  to  determine  their  characteristics  in  such 
terms  that  the  kind  of  plate  or  film  may  thereafter  be  speci- 
fied and  selected  on  the  basis  of  purely  laboratory  tests. 
Specification  must  be  made  in  terms  of  the  ordinary  sensi- 
tometric  constants  of  the  photographic  emulsion — its  speed, 
contrast,  fog,  development  factor,  its  color  sensitiveness,  its 
ability  to  render  fine  detail,  and  its  grosser  physical  properties 
such  as  hardness  and  shrinkage. 

Sensitometry. — The  most  generally  used  system  of 
sensitometry  is  that  of  Hurter  and  Driffield,  commonly 
referred  to  as  the  "H  &  D."  By  this  system,  in  order  to 
determine  the  characteristics  of  a  given  photographic  plate, 
it  is  necessary  to  take  a  series  of  graduated  exposures,  a 
standard  illumination  of  the  plate  being  varied  in  known 
amount  by  a  rapidly  rotating  disc  cut  to  a  series  of  different 
openings,  or  by  some  other  suitable  means.  The  negative 
thus  obtained  is  developed  in  a  standard  developer  for  a 
definite  time,  at  a  fixed  temperature,  and  is  then  measured 
for  transmission  on  a  photometer.  The  following  terms  are 
defined  and  used  in  plotting  the  results: 

„,  _      intensity  of  light  transmitted       / 

Transparency  =  T  =  -      — £r j% — rj — .  ..  ,  .       =  -j- 

mtensity  of  incident  light  I0 

227 


228        AIRPLANE  PHOTOGRAPHY 

_      _     intensity  of  incident  light      _  h  _  1 
~  intensity  of  transmitted  light  ~  7  ~  ~f 
Density  =  D  =  -  logio  T  =  logioO 

Hurter  and  Driffield  pointed  out  that  a  negative  would 
give  a  true  representation  of  the  differences  in  the  light  and 
shade  of  the  object  if  it  reproduced  these  differences  by 
equivalent  differences  in  opacity.  This  is  equivalent  to 
stating  that  if  the  densities  are  plotted  against  the  logarithms 
of  the  corresponding  exposures,  a  straight  line  should  be 
obtained  at  45  degrees  to  the  axis  of  exposure  times.  If  the 
line  is  at  another  angle  the  opacities  of  the  negative  will  be 
proportional  to  the  brightness  of  the  object  photographed, 
but  the  contrast  will  be  different. 

A  typical  H  &  D  plot  is  shown  in  Fig.  103.  It  will  be 
noted  that  two  curves  are  shown.  These  are  obtained  with 
different  developments,  and  illustrate  the  fact  that  the  con- 
trast or  proportionality  between  exposure  .differences  and 
opacity  differences  is  a  matter  of  time  of  development.  Each 
of  these  curves  exhibits  certain  characteristics  which  are 
common  to  all  made  in  this  way.  There  is  primarily  a 
straight  line  portion,  where  opacities  are  proportional  to 
illumination.  This  is  commonly  called  the  region  of  correct 
exposure.  The  slope  of  this  straight  line  portion  —  the  ratio  of 

~is  tne  development  factor,    commonly   denoted 


by  "r"  a  gamma  of  unity  denoting  exact  tone  rendering. 
Below  the  region  of  correct  exposure  is  a  "toe,"  or  region  of 
smaller  contrast,  called  the  region  of  under  exposure.  Above 
the  correct  exposure  region  is  another  where  the  opacity 
approaches  constancy  (afterwards  decreasing  or  "revers- 
ing"), called  the  region  of  over  exposure. 

The  speed  of  a  plate  on  the  H  &  D  scale  is  given  by  the 
intersection  of  the  straight  line  portion  of  the  characteristic 
curve  when  produced,  with  the  exposure  axis,  This  inter- 


PHOTOGRAPHIC  EMULSIONS 


section  point,  called  the  inertia,  is  the  same  irrespective  of 
the  time  of  development,  as  is  shown  in  Fig.  103.  The  numer- 
ical value  of  the  speed  is  obtained  by  dividing  34  by  the  iner- 
tia, when  the  exposure  is  plotted  in  candle-meter-seconds. 

If  a  plate  is  developed  until  no  more  density  and  contrast 
can  be  obtained,  its  development  factor  is  then  ^m,  (gamma 


3  and  6  mlnutos 


FIG.  103. — Typical  characteristic  curves  of  photographic  plate. 

infinity),  and  the  larger  this  is  the  more  a  plate  can  be  forced 
in  development.  If  the  plate  fogs  in  its  unexposed  portions 
this  fog  is  measured  and  recorded  in  density  units  along  with 
the  other  constants.  The  speed  of  development  is  repre- 
sented by  the  velocity  constant,  commonly  symbolized  by  K. 
The  length  of  the  straight  line  portion  determines  the 
latitude  of  the  plate,  or  the  range  of  permissible  exposures  to 
secure  a  "perfect  negative."  Thus  if  we  assume  that  an 


230        AIRPLANE  PHOTOGRAPHY 

object  has  a  range  of  brightness  of  1  to  30,  then  a  plate  with 
a  straight  line  characteristic  extending  over  a  range  of  1  to  120 
would  have  a  latitude  of  l-ff  or  4.  That  is,  the  exposure  could 
be  as  much  as  four  times  the  necessary  one,  and  still  give  the 
sameresultonasufficientlyexposedprint.  Ifthelatitudeofthe 
plate  is  too  small,  the  shadows  will  fall  in  the  under  exposure  re- 
gion, the  high-lights  in  the  over  exposure  portion  of  the  char- 
acteristic curve,  with  consequent  poor  rendering  of  contrasts. 

Criteria  of  Speed. — In  airplane  photography  speed  is  of 
paramount  importance,  but  great  care  must  be  exercised  to 
insure  that  all  the  factors  are  considered  which  can  contribute 
toward  yielding  the  desirable  pictorial  quality  in  the  brief 
exposure  which  alone  is  possible  from  the  moving  plane.  A 
"fast"  plate  on  the  H  &  D  scale  is  not  necessarily  suitable 
for  aerial  work,  when  we  remember  that  accentuation  of 
natural  contrast  is  desirable,  particularly  under  hazy  condi- 
tions. For,  as  is  shown  in  Fig.  104,  it  is  a  common  character- 
istic of  "fast"  plates  to  have  comparatively  small  latitude 
and  low  contrast  at  their  maximum  development. 

It  is  to  be  noted  that  the  Hurter  and  Driffield  measure  of 
speed  is  bound  up  with  the  idea  of  correct  tone  rendering  and 
with  the  use  of  the  straight  line  portion  of  the  characteristic 
curve.  Other  criteria  of  speed  exist.  For  instance,  the  expos- 
ure necessary  to  produce  a  just  noticeable  action  (threshold 
value);  and  the  exposure  necessary  to  give  a  chosen  useful 
density  in  the  high-lights  when  development  is  pushed  to  the 
limit  set  by  the  growth  of  fog. 

As  has  already  been  pointed  out,  correct  tone  rendering 
is  not  necessary  or  even  indicated  as  desirable  in  aerial  views. 
It  is,  moreover,  a  matter  of  experience  that  the  majority  of 
aerial  exposures  with  existing  plates  fall  in  the  "under 
exposure  "  period,  where  contrasts  with  normal  development 
are  less  than  in  the  subject.  This  being  the  case,  the  problem 


PHOTOGRAPHIC  EMULSIO'NS     231 


H.&.  D.  Characteristic  Curves 


.4 


Log  Exposure 
FIG.  104. — Characteristic  curves  of  fast  and  slow  plates,  developed  to  maximum  contrast. 

is  to  select  not  necessarily  a  fast  plate,  by  the  H  &  D  criterion, 
but  a  plate  which  will  develop  up  workable  densities  in  the 
under  exposure  region.  A  plate  of  medium  speed  will  some- 


AIRPLANE  PHOTOGRAPHY 

times  develop  to  greater  densities  in  the  short  exposure 
region,  if  development  is  forced,  than  will  a  fast  plate.  The 
contrast  in  the  normal  exposure  region  will  be  excessive,  but 
this  is  of  no  significance  if  no  exposure  falling  in  this  region 
is  present  on  the  plate. 

In  addition  to  its  capacity  for  developing  density,  the 
plate  should  have  as  low  a  threshold  as  possible,  thus  meet- 
ing to  some  extent  the  requirements  of  both  the  alternative 
criteria  of  speed  given  above.  At  the  same  time  it  is  true 
that  low  threshold  and  good  density  for  short  exposures 
are  not  to  be  found  in  really  slow  plates.  Consequently, 
while  high  speed,  as  ordinarily  understood,  is  undoubtedly 
the  first  requirement,  we  may  expect  the  complete  specifica- 
tion for  the  best  aerial  plate  to  be  a  rather  complicated  thing, 
describing  the  characteristics  of  a  workable  "toe"  of  the 
curve,  in  terms  of  which  several  (e.g.,  contrast  and  speed)  are 
derived  from  another  and  quite  different  exposure  region. 

Effect  of  Temperature  on  Plate  Speed. — It  has  been  found 
by  Abney  and  Dewar  that  very  low  temperatures  materially 
decrease  the  speed  of  photographic  emulsions.  This  decrease 
may  amount  to  as  much  as  50  per  cent,  in  the  temperature 
range  from  30  degrees  Centigrade  above  zero  to  30  degrees 
below  zero,  which  is  the  range  over  which  aerial  photographic 
operations  will  have  to  be  carried  on  in  war-time.  This 
effect  has  not  been  at  all  fully  studied,  and  it  is  not  known 
whether  it  is  general  or  only  found  in  certain  kinds  of  plates. 
The  remedy  indicated  is  to  provide  means  for  heating  the 
plates  or  films  when  low  temperatures  are  encountered.  This 
is  fairly  easy  in  film  cameras,  or  in  plate  cameras  like  the 
deRam,  where  the  entire  load  of  plates  is  carried  in  the 
camera  body.  Plates  carried  in  magazines  present  a  more 
difficult  problem.  The  heating  coil  incorporated  in  the 
German  cameras  is  perhaps  partly  for  this  purpose. 


PHOTOGRAPHIC  EMULSIONS     233 

Color  Sensitiveness. — Complete  specifications  for  an 
aerial  plate  cannot  be  made  solely  on  the  basis  of  its  speed, 
contrast,  latitude,  threshold,  and  other  sensitometric  values 
which  have  to  do  only  with  the  intensity  of  the  light  acting 
on  it.  These  in  general  apply  to  photography  from  low  alti- 
tudes, where  the  illumination  and  natural  contrast  of  the 
subject  are  the  only  factors  to  consider.  When  higher  alti- 
tudes are  reached  the  interposition  of  haze  decreases  the 
already  deficient  contrast,  calling  either  for  the  development 
of  more  contrast  in  the  plate*  or  for  the  use  of  color  filters 
to  cut  out  the  action  of  the  blue  and  violet  light  predominant 
in  haze.  Along  the  lines  discussed  in  the  last  section,  it  is 
not  surprising  to  find  that  some  plates  are  better  than  others 
for  bringing  out  gradations  masked  by  haze,  even  though  no 
filters  are  used  and  though  the  plates  are  similar  in  color 
sensitiveness.  But  the  limitations  to  securing  contrast  by 
manipulating  the  characteristic  curve  of  the  plate  are  soon 
reached,  and  it  becomes  necessary  to  resort  to  haze-piercing 
color  filters,  used  with  color  sensitive  plates. 

Roughly,  two  general  types  of  color  sensitive  emulsions 
may  be  distinguished:  first,  those  in  which  sensitiveness  to 
green  and  yellow  is  added  to  the  natural  blue  sensitiveness, 
and  second,  those  sensitive  in  a  useful  degree  to  all  colors  of 
the  spectrum.  The  former  are  called  iso-  or  ortho-chromatic, 
the  latter  panchromatic  emulsions.  Spectrograms  exhibiting 
the  distribution  of  sensitiveness  throughout  the  spectrum 
for  several  representative  plates  are  shown  in  Fig.  105. 
Orthochromatic  plates  are  adequate  for  use  with  light  yellow 
filters  and  have  the  slight  practical  working  advantage  that 
they  can  be  handled  by  red  light.  Panchromatic  plates  are 
necessary  for  use  with  dark  orange  or  red  filters.  They  must 
be  handled  in  total  darkness  or  in  an  exceedingly  faint  blue- 
green  light,  taking  advantage  of  the  common  drop  in  sensi- 


FIG.  105. — Spectrograms  of  representative  photographic  plates:  a,  ordinary  plate;  b,  ortho- 
chromatic  plate;  c,  specially  green-sensitive  plate;  d,  red  sensitive  plate,  insensitve  to  green;  e,  pan- 
chromatic plate;  /,  specially  red-sensitive  panchromatic  plate. 


PHOTOGRAPHIC  EMULSIONS     235 

bility  in  that  region  of  the  spectrum.  Plates  can,  indeed,  be 
sensitized  for  the  red  alone,  leaving  a  gap  of  almost  complete 
insensibility  in  the  green,  as  shown  in  the  fourth  spectrogram 
of  Fig.  105.  When  used  with  a  yellow  filter  these  plates 
behave  as  do  panchromatic  plates  with  a  red  filter. 

A  rougher  idea  of  color  sensitiveness  than  is  given  by 
spectrograms  is  furnished  by  the  tri-color  ratio,  which  is  the 
ratio  of  exposure  times  necessary  with  white  light  to  give 
equal  photographic  action  through  a  certain  set  of  red,  green 
and  blue  filters,  expressed  in  terms  of  the  blue  exposure  as 
unity.  In  an  excellent  panchromatic  plate  the  three  expos- 
ures would  be  equal.  In  an  orthochromatic  plate  the  red 
exposure  will  be  too  large  to  be  figured.  In  interpreting  either 
spectrograms  or  tri-color  ratios  care  must  be  taken  that  the 
absolute  exposures  necessary  are  known.  Thus  a  relatively 
high  red  sensitiveness  may  mean  merely  low  absolute 
blue  sensitiveness. 

Two  methods  are  used  in  imparting  color  sensitiveness. 
Either  the  sensitizing  dye  is  incorporated  in  the  plate  emul- 
sioh  before  it  is  flowed;  or  the  plate  is  bathed  in  a  dye 
solution  not  long  before  using.  The  latter  method  gives 
higher  color  sensitiveness  but  poorer  keeping  quality,  and 
is  not  a  practical  method  for  field  operations.  Greatly 
enhanced  sensibility  may  be  given  by  treatment  with  am- 
monia, but  this  again  is  a  method  for  laboratory  rather 
than  field  use. 

Resolving  Power. — A  question  which  arises  in  connection 
with  all  photography  of  detail  is  the  size  of  the  grain  of  the 
photographic  emulsion.  Dependent  on  the  size  of  the  grain 
is  the  resolving  power,  or  ability  to  separate  images  of  closely 
adjacent  objects.  This  varies  with  the  speed,  fast  plates 
being  of  coarser  grain  than  slow  ones;  with  the  exposure;  and 
with  the  method  and  time  of  development.  In  general,  it 


236        AIRPLANE  PHOTOGRAPHY 

may  be  said  that  the  resolving  power  of  the  plate  does  not 
enter  practically  into  aerial  work,  because  the  resolving 
power  of  all  plates  so  far  found  usuable  corresponds  to  a 
smaller  distance  than  the  size  of  a  point  image  as  limited  by 
the  performance  of  the  camera  lens  and  the  speed  of  the 
plane.  Remembering  that  -^o  mm.  is  a  fair  value  for  the 
size  of  a  point  image  as  rendered  by  the  lens,  the  role  of  plate- 
resolving  power  is  shown  by  consideration  of  the  following 
table.  Resolving  powers  are  given  in  terms  of  lines  to  the 
millimeter  just  separable. 

Emulsion.  Resolving  Power. 

Seed  Graflex  25 

Eastman  Aerial  Film  37 

Hammer  Ortho  44 

Cramer  Isonon  48 

Cramer  Spectrum  Process  57 

Eastman  Portrait  Film  61 

Tabulation  of  Requirements  for  Aerial  Emulsions. — 
In  terms  of  the  sensitometric  quantities  just  discussed  the 
general  requirements  for  aerial  plates  may  be  listed  as  f ollows : 

1.  Speed.  The  speed  usually  connected  with  the  contrast 
and  density  required  for  the  exposure  times  available  is 
about  150  H  &  D.     Faster  plates  in  general  have  too  low 
contrast,  but  the  highest  speed  that  will  give  the  necessary 
contrast  is  desired. 

2.  Contrast.    The  contrast  capable  of  development  with- 
out fog  should  be  from  1.5  to  2.     This  contrast  should  be 
produced  by  light  of  daylight  quality,  and,  in  orthochromatic 
and  panchromatic  plates,  with  the  yellow  or  orange  filters 
intended  to  be  used  with  them.     This  contrast  means  a 
gamma  infinity  approaching  2.5. 

3.  Speed  of  development.    A  gamma  of  nearly  2  should  be 
developed  in  2J/2  minutes  at  20  degrees  C.  in  the  developers 
recommended  below. 


PHOTOGRAPHIC  EMULSIONS     237 

4.  Fog.    Not  over  .25  for  this  degree  of  development,  and 
not  over  .40  for  six  minutes  development. 

5.  Color  sensitiveness.    This  should  in  general  be  as  high 
as  possible.     In  terms  of  certain  representative  filters  (de- 
scribed in  a  subsequent  chapter)  color  sensitiveness  should 
be  such  that  with  the  white  light  speed  above  specified  the 
relative  exposures  through  the  filters  shall  not  be  greater 
than  as  follows: 

No  filter        Aerol  Aero  2  #21         #23a    #25 

Panchromatic  plate  1  3  4.5  7  9       12 

Ortho  plate  1  2.5  3.5  6 

Relative  Behavior  of  Plates  and  Films. — The  advantages 
of  film  from  the  standpoint  of  weight  and  bulk  have  been 
discussed  in  connection  with  aerial  cameras.  Were  there  no 
other  considerations  film  would  unquestionably  be  the  most 
appropriate  medium  for  aerial  photography.  There  is,  how- 
ever, the  question  of  ease  of  handling,  to  be  treated  in  a 
subsequent  chapter,  and  the  question  whether  the  purely 
photographic  characteristics  of  film  are  satisfactory.  Can 
the  same  speed,  contrast,  and  color  sensitiveness  be  obtained 
on  film  as  on  glass?  Is  the  picture  so  obtained  as  permanent 
or  reliable  as  the  plate  image? 

It  must  be  confessed  that  up  to  the  present  emulsions 
on  film  have  not  proved  the  equal  of  those  on  glass.  It  has 
been  found  by  emulsion  manufacturers  that  the  same  emul- 
sion flowed  on  film  and  on  glass  gives  better  quality  on  the 
glass.  Emulsions  specially  prepared  for  film  fall  somewhat 
short  of  the  best  plate  emulsions.  It  has  also  been  found 
harder  to  color-sensitize  film,  and  to  insure  good  keeping 
quality  in  the  color  sensitized  product. 

In  addition  to  the  question  of  photographic  quality  there 
arises  the  matter  of  shrinkage  and  distortion.  These  are 
negligible  with  plates,  but  are  a  more  or  less  unknown  quan- 
tity in  film.  Irregular  shrinkages  of  as  much  as  two  per  cent. 


238        AIRPLANE  PHOTOGRAPHY 

are  found  on  experiment.  This  defect,  of  course,  would  be  an 
obstacle  only  in  exact  mapping  work. 

Positype  Paper. — The  need  sometimes  arises  in  military 
operations  to  secure  prints  ready  for  examination  within  a 
few  minutes  after  the  receipt  of  the  negatives.  Even  the 
15  or  20  minutes  within  which  a  negative  can  be  developed 
and  a  wet  print  taken  may  be  considered  too  long.  While 
such  occasions  are  probably  more  apt  to  occur  in  popular 
magazine  stories  than  in  actual  warfare,  it  is  important  to 
have  available  methods  of  producing  prints  with  an  absolute 
minimum  of  delay.  This  need  is  met  to  some  degree  by  a 
direct  print  process,  commercially  exploited  under  the  name 
of  "Positype." 

In  this  process  the  exposure  is  made  directly  on  a  sensi- 
tized paper  or  card,  which  is  developed,  the  image  dissolved 
out,  the  residue  exposed,  and  again  developed;  thus  furnish- 
ing a  positive  picture  (reversed  right  and  left).  The  time 
necessary  to  develop  a  print  ready  for  examination  need 
not  be  more  than  three  minutes.  Only  a  single  print  is 
available,  but  this  is  all  that  would  be  called  for  under  the 
extreme  conditions  suggested.  If  later,  copies  are  desired 
they  may  be  made  by  the  same  process. 

Plates  and  Films  Found  Satisfactory  for  Aerial  Work. — 
The  following  plates  and  films  have  been  found  particularly 
good  for  aerial  photography.  The  list  is  not  intended  to  be 
complete.  Furthermore,  it  may  be  expected  to  be  soon  super- 
seded, as  the  efforts  of  various  manufacturers  are  directed 
toward  developing  special  aerial  photographic  plates. 

Among  orthochromatic  plates :  The  Cramer  Commercial 
Isonon,  the  Jougla  Ortho. 

Among  panchromatic  plates:  The  Ilford  Special  Pan- 
chromatic, the  Cramer  Spectrum  Process. 

Film:   Ansco  Speedex,  Eastman  Aero. 


CHAPTER  XIX 
FILTERS 

The  Function  of  Filters  in  Aerial  Photography. — The  use 

of  color  screens  or  filters  has  been  very  common  in  ordinary 
landscape  photography,  for  the  purpose  of  securing  approx- 
imately correct  renderings  of  the  brightnesses  of  colored 
objects.  Plates  of  the  non-color-sensitive  type  have  their 
maximum  of  sensitiveness  in  the  blue  of  the  spectrum  (Fig. 
105)  and  in  consequence  blue  skies  photograph  as  white, 
while  other  colors  are  likewise  reproduced  on  a  totally  wrong 
scale.  Filters  for  correct  brightness  rendering  are  calculated 
for  a  given  color  sensitive  plate  so  that  the  resultant  reaction 
to  the  light  of  the  spectrum  copies  the  sensitiveness  of  the 
eye,  which  is  greatest  in  the  yellow-green.  Such  filters  for 
use  with  the  common  orthochromatic  plates  are  of  a  general 
yellow  color. 

Filters  for  aerial  work  are  meant  to  serve  quite  a  different 
purpose.  Correct  tone  or  color  rendering  is  of  quite  second- 
ary importance  to  another  use  of  filters,  namely,  to  cut  or 
pierce  aerial  haze.  It  is  quite  a  matter  of  accident  that  the 
same  general  color  of  filter  is  called  for  both  to  give  correct 
color  rendering  and  to  pierce  aerial  haze,  namely,  yellow.  Yet 
on  closer  analysis  it  is  found  that  quite  different  types  of 
yellow  filter  are  demanded,  spectroscopically  considered. 

Figure  106  (Ki  and  K2)  shows  the  spectral  transmission 
curves  of  the  Wratten  KI  and  K2  filters,  intended  for  correct 
color  rendering  with  orthochromatic  plates.  The  absorption 
increases  gradually  toward  the  blue.  In  the  same  figure  is 
shown  on  an  arbitrary  scale  the  spectroscopic  character  of 
typical  haze  illumination,  increasing  in  brightness  inversely 

239 


240 


AIRPLANE  PHOTOGRAPHY 


as  the  fourth  power  of  the  wave-length,  that  is,  with  great 
rapidity  in  the  blue  and  violet.  It  is  evident  from  this  that 
a  much  more  abrupt  absorption  than  that  of  the  KI  or  K2 
filter  is  desirable,  because  in  the  green  of  the  spectrum  the 
haze  light  is  comparatively  weak,  and  more  will  be  lost  by 
any  absorption  in  this  region  through  decreasing  useful 


jo  a 


Violet:       Blue,  Green         Yellow   Oranye 

FIG.  106.  —  Characteristics  of  various  filters. 


70/4 


photographic  action  than  will  be  gained  by  cutting  out  the 
haze.  This  latter  consideration  is  important.  The  use  of 
any  filter  means  an  increase  of  exposure;  the  use  of  yellow 
filters  multiplies  it  several  times.  Careful  experiment  has 
shown  that  no  filter  of  depth  less  than  K  li,  to  use  the  Wratten 
filters  as  a  basis  for  discussion,  are  of  real  value  in  haze 
piercing.  The  filter  ratio,  or  ratio  of  exposures  with  and 
without  filter,  is  4.7  for  the  K  li  with  the  Cramer  Isonon 


FILTERS  £41 

plate — a  figure  which  shows  the  importance  of  securing  the 
necessary  haze-piercing  character  with  the  minimum  absorp- 
tion of  useful  photographic  light. 

Practical  Filters. — Since  the  character  of  the  absorption 
of  the  "K"  filters  is  not  all  that  could  be  desired,  new  filters, 
both  of  dyed  gelatin  and  of  glass,  have  been  produced.  The 
glass,  a  Corning  product  having  a  very  sharp-cut  absorption, 
has  not  yet  been  produced  on  a  commercial  scale  with  the 
high  transparency  in  green,  yellow  and  red  that  selected 
samples  have  shown.  The  United  States  Air  Service  has 
adopted  filters  of  a  new  dye,  called  the  EK,  from  the  name 
of  the  company  in  whose  laboratory  it  was  produced.  These 
filters  are  standardized  in  two  depths  of  staining,  called  the 
"Aero  No.  1"  and  "Aero  No.  2."  Their  spectral  transmis- 
sion curves  appear  in  Fig.  106,  along  with  those  of  certain 
darker  filters  useful  only  with  panchromatic  plates  for 
exceptionally  heavy  haze.  The  characteristic  of  these  Aero 
filters  is  their  great  transparency  through  all  the  spectrum 
except  the  blue,  whereby  the  greatest  haze-cutting  action  is 
attained  together  with  a  low  filter  factor.  The  filter  factors 
of  the  Aero  No.  1  and  No.  2  with  Cramer  Isonon  plates  are 
3  and  5,  respectively. 

Effects  Secured  by  the  Use  of  Filters. — The  efficiency  of 
yellow  filters  for  haze-cutting  is  best  shown  by  photographs 
taken  at  high  altitudes  with  filters  and  without.  Such  illustra- 
tions are  given  in  Figs.  107  and  108,  where  the  first  photograph 
is  one  taken  at  10,000  feet  without  a  filter,  the  second  taken 
at  the  same  altitude  under  the  same  conditions,  but  with 
an  orange  filter.  Both  are  on  panchromatic  plates,  and  it 
will  be  seen  that  even  with  these  plates  the  filter  makes  all 
the  difference  between  a  useless  and  a  useful  picture.  But 
it  must  be  clearly  understood  that  the  difference  here  lies 
between  a  plate  sensitive  chiefly  in  the  blue  and  violet,  and 

16 


AIRPLANE  PHOTOGRAPHY 

a  plate  affected  only  by  the  yellow,  orange  and  red.  The 
difference  is  not  between  what  the  eye  sees  and  wThat  a  plate 
with  a  filter  sees,  as  is  sometimes  supposed.  As  shown  in 
Fig.  108,  a  filter  enables  the  plate  to  photograph  through  the 
haze  between  clouds,  but  not  through  the  clouds  themselves. 
In  general,  no  filter  and  plate  combination  which  is  feasible 


FIG.  107. — A  photograph  taken  at  10,000  feet,  without  a  filter. 

for  aerial  exposures  is  capable  of  showing  more  than  the  eye 
can  see  if  yellow  or  orange  goggles  are  worn.  To  do  this  it 
would  be  necessary  for  the  photographic  action  to  take  place 
by  deep  red  or  infra-red  light,  which  would  demand  expos- 
ures now  out  of  the  question. 

Filters  are  almost  always  necessary  in  photographing 
from  high  altitudes  or  in  making  distant  obliques.  At  times, 
particularly  after  a  heavy  rain,  the  air  is  clear  enough  so 


FILTERS 


243 


that  filters  may  be  dispensed  with.  Clearing  weather  was 
therefore  chosen  whenever  possible  for  making  obliques  of 
the  battle  front. 

Filters  for  the  Photographic  Detection  of  Camouflage. — 
In  the  photographic  as  in  the  visual  detection  of  camouflage, 
the  problem  is  to  differentiate  colors  which  ordinarily  look 


FIG.  108. — Photograph  taken  at  same  time  and  over  same  neighborhood  as  Fig.  107,  but  with  an 

orange  filter. 

alike,  but  which  are  actually  of  different  color  composition. 
Particularly  important  are  the  differences  between  natural 
foliage  greens  and  the  paints  used  to  simulate  them.  If  these 
differ  in  their  reflection  spectra,  a  proper  choice  of  filter  will 
show  up  the  two  greens  as  markedly  different.  Two  kinds 
of  difference  may  be  produced ;  either  the  two  colors  may  be 
changed  in  relative  brightness,  or  they  may  be  altered  in 


244        AIRPLANE  PHOTOGRAPHY 

hue.  Thus  foliage  green,  due  to  its  possessing  a  reflection 
band  in  the  red  of  the  spectrum,  which  is  absent  in  most 
pigments,  may  be  made  to  appear  red  while  the  camouflage 
remains  green  or  turns  black.  Filters  which  cause  changes 
of  color  are  of  course  of  no  use  for  photographic  detection 
of  camouflage,  since  the  photographic  image  is  colorless. 
Brightness  differences  are  alone  available. 

Those  same  filters  which  have  been  worked  out  primarily 
for  producing  brightness  differences  in  visual  detection  of 
camouflage  could  be  used  photographically,  provided  the 
plates  employed  were  color  sensitive,  and  were  as  well 
screened  to  imitate  the  sensibility  of  the  eye.  But  the  most 
useful  visual  filters  are  those  causing  color  differences  to 
appear;  more  than  this,  the  visual  camouflage  detection  filters 
as  a  class  have  low  light  transmissions,  so  that  their  useful- 
ness in  photography  is  doubtful.  Little  work  has  actually 
been  done  with  camouflage  detection  filters  for  photography. 
Yet  in  spite  of  this  photography  has  been  of  real  service  in 
this  form  of  detective  work.  Its  utility  for  the  purpose  comes 
from  the  fact  that  the  natural  sensitiveness  of  the  plate  to  blue, 
violet  and  invisible  ultra-violet  acts  to  extend  the  range  of 
the  spectrum  in  which  differences  between  identical  and 
merely  visually  matched  colors  may  be  picked  up.  Conse- 
quently the  plain  unscreened  plate  has  proved  a  very  effi- 
cient camouflage  detector — so  efficient  in  fact  that  all  camou- 
flage materials  have  had  to  be  subjected  to  a  photographic 
test  before  acceptance.  Fig.  171  shows  how  an  ordinary 
photograph  reveals  the  unnatural  character  of  the  camouflage 
over  a  battery. 

Methodsof  Mounting  and  Using  Filters. — Themost  prim- 
itive way  of  mounting  a  gelatin  filter  is  to  cut  a  disc  from  a 
sheet  of  dyed  gelatin  and  insert  it  between  the  components  of 
the  lens.  For  this  purpose  the  gelatin  must  be  perfectly  flat, 


FILTERS  245 

which  is  insured  by  its  method  of  preparation  and  test.  One 
disadvantage  of  this  method  is  that  the  filter  can  be  inserted 
and  removed  only  upon  the  ground.  It  is  less  satisfactory 
the  larger  the  diameter  of  the  lens,  and  the  wastage  of  filters 
due  to  insertion  and  removal  is  apt  to  be  high.  The  camera 
should  be  refocussed  after  filters  of  this  kind  are  inserted. 

Glass  filters,  ground  optically  true,  or  gelatin  filters, 
mounted  between  optically  flat  glass  plates,  are  the  most 
convenient  and  satisfactory.  They  may  be  mounted  in 
circular  cells  to  screw  or  attach  by  bayonet  catches  to  the 
front  of  the  lens.  Or  they  may  be  mounted  in  rectangular 
frames  to  slide  into  transverse  grooves  in  the  camera  body. 
Fig.  44  shows  the  mount  of  this  latter  form  adopted  in  the 
larger  United  States  Air  Service  cameras.  This  is  partic- 
ularly convenient  if  it  is  desired  to  insert  or  change  the 
filter  while  in  the  air — a  practice  not  generally  considered 
feasible  in  war  work  with  the  photographically  inexperienced 
observer,  but  likely  to  be  common  with  the  employment  of 
skilled  photographers  for  peace-time  aerial  photography. 

German  cameras  are  reported  in  which  the  glass  filter  is 
carried  behind  the  lens,  on  a  lever  which  also  carries  a  clear 
glass  plate  of  the  same  thickness,  to  be  thrown  in  when  no 
filter  is  needed,  thus  maintaining  the  focus.  The  perform- 
ance of  the  lens  will  be  impaired  by  this  scheme,  unless  it  is 
specially  calculated  to  offset  the  effect  of  the  glass  introduced 
in  the  path  of  the  rays  behind  the  lens — optically  true  glass 
has  no  effect  on  definition  if  placed  in  front  of  the  lens.  Glass 
filters  may  also  be  placed  in  close  contact  with  the  plate 
or  film,  in  which  case  they  must  be  much  larger,  but  do  not 
need  to  be  of  as  good  optical  quality. 

Self=screening  Plates. — Mention  must  be  made  of  a 
quite  different  mode  of  realizing  the  filter  idea,  a  method 
available  where  the  sensitive  plate  is  always  to  be  used  with 


246        AIRPLANE  PHOTOGRAPHY 

a  filter.  This  is  to  incorporate  a  yellow  dye  in  the  gelatin 
of  the  plate  itself.  The  dye  must  be  one  which  has  no  direct 
chemical  effect  on  the  plate,  but  which  acts  simply  as  a 
coloring  agent  for  the  gelatin.  "  Self  -screening "  plates,  as 
they  are  called,  have  been  produced  by  the  use  of  the  dye 
called  "filter  yellow"  and  have  found  some  use  in  ortho- 
chromatic  photography.  They  effect  a  useful  saving  of  light 
through  the  elimination  of  the  reflection  losses  at  the  sur- 
faces of  glass  and  gelatin  filters.  The  filtering  action  of  the 
dye  in  the  plate  is  somewhat  different  from  its  ordinary 
one,  since  the  deeper  portions  of  the  sensitive  film  are  sub- 
ject to  greater  action  than  the  surface,  and  this  tends  to 
diminish  contrast. 


CHAPTER  XX 
EXPOSURE  OF  AERIAL  NEGATIVES 

The  principal  factors  governing  the  length  of  exposure  in 
the  airplane  camera  have  already  been  discussed  under 
various  headings.  These  are  briefly,  the  nature  of  the  aerial 
landscape,  the  practically  attainable  lens  apertures,  the 
form  of  the  camera  support,  the  speed  of  the  plane,  and  the 
characteristics  of  plates,  films  and  filters.  It  is  convenient 
however,  to  re-assemble  this  information  in  one  place,  in 
such  form  as  to  apply  to  the  practical  problem  of -determin- 
ing the  exposure  to  be  given  in  any  specific  case. 

Limitations  to  Exposure. — In  the  ordinary  photography 
of  stationary  objects,  exposure  is  a  variable  entirely  at  the 
operator's  command.  Plates  of  any  speed  may  be  selected, 
so  that  attention  may  be  focussed  on  latitude,  color  sensi- 
tiveness, and  other  tone  rendering  characteristics.  The 
exposure  may  be  made  of  a  length  sufficient  to  insure  all  the 
useful  photographic  action  lying  in  the  "correct  exposure" 
portion  of  the  sensitometric  curve.  The  exposure  ratio  of 
any  filter  it  is  desired  to  use  is  a  matter  of  indifference — its 
effect  on  color  rendering  need  alone  be  considered. 

Airplane  photography  is  sharply  distinguished  from 
ground  "still"  photography  by -its  severe  limitations  as  to 
the  amount  of  hthe  exposure.  The  actual  duration  is  defi- 
nitely restricted  by  the  high  speed  of  the  plane.  In  peace 
work  this  can  be  offset  in  part  by  using  slower  planes  or  by 
flying  against  the  wind.  The  practical  limitation  to  ifa 
second,  set  by  war-time  requirements  as  to  definition  of  fine 
detail,  may  be  increased  to  ^  of  a  second,  or  even  more, 
where  mapping  of  grosser  features  is  the  object.  A  common, 

247 


248        AIRPLANE  PHOTOGRAPHY 

but  entirely  avoidable  limitation,  is  that  due  to  vibration 
of  the  camera.  By  proper  mounting  this  may  be  entirely 
overcome,  leaving  the  ground  speed  of  the  plane  the  only 
source  of  exposure-limiting  movement.  The  amount  of  light 
reaching  the  plate  constitutes  a  primary  factor  in  exposure, 
and  this  is  a  matter  of  lens  aperture.  Generally,  lens  aperture 
is  smaller  the  larger  the  plate  required  to  be  covered,  and  the 
greater  the  focal  length.  Because  of  their  larger  aperture, 
the  short-focus  lenses  which  will  be  favored  for  peace-time 
large-area  mapping  will  permit  more  and  longer  working 
days  than  have  been  the  rule  in  long-focus  war  photography. 
The  necessary  use  of  filters,  particularly  at  the  high  altitudes 
which  would  be  chosen  in  mapping,  in  order  to  economize 
in  the  number  of  flights  needed  to  cover  a  given  area,  intro- 
duces an  inevitable  decrease  in  the  amount  of  light  available 
at  the  plate,  as  compared  with  surface  photography  under 
the  same  illuminations. 

Broadly  speaking,  it  may  be  said  that  all  the  demands 
made  in  reference  to  aerial  photographic  exposure  work  are  to 
decrease  the  amount  of  light  reaching  the  plate.  Any  surplus 
offered,  as  by  the  midsummer  noon-day  sun,  must  be  immedi- 
ately snapped  up,  either  by  decreasing  the  exposure  to  get 
greater  sharpness,  or  by  introducing  filters  to  get  greater 
photographic  contrast.  The  absolute  exposure  of  the  plate 
tends  to  be  kept  at  the  irreducible  minimum.  As  already 
stated,  it  lies,  with  present  photographic  materials,  on  the 
"toe"  of  the  "H  &  D"  curve,  just  reaching  up  into  the 
straight  line  portion. 

Estimation  of  Exposure. — According  to  the  foregoing 
argument  the  problem  of  estimating  an  aerial  exposure 
resolves  itself  largely  into  one  of  deciding  how  short  this 
may  be  made.  Or,  if  the  light  is  strong,  whether  it  is  suffi- 
cient so  that  a  filter  may  be  introduced  without  demanding 


EXPOSURE  OF  NEGATIVE'S      249 

more  than  the  TO~O  second  or  thereabouts  which  is  dictated 
by  the  motion  of  the  plane. 

Deciding  upon  exposures  in  the  field  has  been  largely 
a  matter  of  experience  and  judgment.  A  majority  of  the 
cameras  in  use  during  the  war  were  not  furnished  with  shut- 
ters calibrated  in  definite  speeds.  Consequently,  the  sergeant 
upon  whom  the  decision  usually  devolved  became  a  store- 
house of  knowledge  as  .to  the  slit  widths  and  tensions  appro- 
priate to  each  individual  camera.  This  knowledge  had  to 
be  acquired  from  the  results  of  actual  photographic  recon- 
naissances, or  from  special  test  flights,  both  of  them  wasteful 
methods.  But  the  chief  objection  to  this  state  of  affairs 
lies  in  the  fact  that  the  knowledge  thus  acquired  is  of  no  use 
to  anyone  else,  nor  is  it  applicable  to  other  types  of  camera. 

The  first  essential  to  placing  exposure  estimation  upon  a 
sound  basis  is  therefore  an  accurate  knowledge  of  shutter 
performances.  Either  the  shutter  speeds  should  be  placed 
upon  the  camera  by  the  manufacturer  and  periodically 
checked,  or  a  regular  practice  should  be  followed  of  calibrat- 
ing shutters,  either  at  a  base  laboratory  or  even  in  the  field. 

Assuming  that  the  speeds  of  all  shutters  are  accurately 
known,  the  process  of  estimating  the  requisite  exposure 
becomes  less  a  matter  of  mere  guesswork  and  more  nearly 
a  matter  of  precision.  For  this  purpose  data  on  the  variation 
of  light  intensity  during  the  day  and  during  the  year  (Figs. 
101  and  102)  should  be  taken  as  a  guide.  These  data  refer 
of  course  to  visual  and  not  to  photographic  light,  but  since 
it  is  always  necessary  to  use  color  filters,  which  make  the 
active  light  of  approximately  visual  quality,  this  is  no  valid 
objection.  The  effects  of  clouds  and  mist  must  of  course  be 
learned  largely  by  experience,  but  with  the  above  daylight 
data  at  hand,  anyone  in  possession  of  definite  information 
on  the  correct  exposure  with  a  given  plate  for  a  known  day 


250 


AIRPLANE  PHOTOGRAPHY 


and  hour  need  not  go  far  wrong  in  estimating  exposures  at 
any  other  time  in  definite  fractions  of  a  second. 

Exposure   data   charts.     Fig.    109   shows   a   chart,   pre- 


FIG.  109. — Chart  showing  aerial  exposures  for  all  times  of  the  day  and  year.  Data  on  basis  of 
F/5.6  lens,  Jougla  orthochromatie  plate,  and  clear  sunlight,  no  filter.  Exposures  to  be  doubled  and 
tripled  for  overcast  and  cloudy  weather. 

pared  in  the  French  service,  indicating  aerial  exposures  for 
all  hours  of  the  day  throughout  the  year.  These  are  for  clear 
sunlight,  for  a  lens  of  aperture  F/5.6  and  for  "ortho"  plates 


EXPOSURE  OF  NEGATIVES 

without  a  filter.  They  are  based  on  what  is  probably  an 
over-estimate  of  the  actual  speeds  given  by  the  French 
shutters.  For  "light"  clouds  the  exposures  are  to  be 
doubled,  for  "heavy"  clouds  quadrupled,  and  for  forests 
and  dark  ground  "lengthened."  Charts  of  this  form  should 
be  extremely  useful,  but  they  were  actually  not  of  great 
service  because  of  the  prevalent  lack  of  knowledge  of  true 
shutter  speeds. 

Exposure  meters.  Aerial  photography  offers  an  excellent 
opportunity  for  the  use  of  exposure  meters,  particularly 
those  of  the  type  in  which  a  sensitive  surface  is  exposed  to 
the  light  for  a  measured  time  sufficient  to  darken  a  pre- 
determined amount.  The  sensitive  paper  of  the  meter  may 
either  be  exposed  from  the  ground  to  the  direct  light  of  sun 
and  sky,  or  from  the  plane  to  the  light  reflected  from  the 
ground.  The  first  method  will  give  figures  subject  to  some 
correction  for  the  character  of  the  ground  to  be  photo- 
graphed— whether  fields,  forests,  or  snow.  The  second 
method  is  to  be  preferred  where  the  shutter  speed  can  be 
adjusted  in  the  air,  according  to  the  indications  of  the 
meter,  or  where  the  filter  can  be  selected  and  put  in  place 
during  flight.  Trials  with  a  commercial  Wynne  exposure 
meter,  used  in  the  latter  manner,  give  as  a  working  figure  an 
exposure  of  .001  second  for  each  4^2  seconds  taken  to  darken 
the  sensitometer  strip  to  match  the  darker  comparison 
patch.  This  relation  applies  to  a  lens  of  aperture  F/4.5,  on 
Cramer  Commercial  Isonon  plates  without  filter. 


CHAPTER  XXI 
PRINTING  MEDIA 

Skilled  photographers  can  examine  a  negative  and  can 
interpret  its  renderings  with  practically  as  much  satisfaction 
as  they  get  from  a  print,  whereby  a  considerable  amount  of 
time  can  be  saved  in  an  emergency.  The  original  glass 
negative  should  always  be  used  when  accurate  measurements 
are  to  be  made.  These  and  a  few  other  cases  constitute  the 
only  use  of  a  negative  apart  from  its  normal  one,  namely, 
for  producing  positive  prints,  usually  in  large  numbers.  The 
commonest  form  of  print  is  on  paper,  although  the  most 
satisfactory  print  from  the  photographic  standpoint  is  the 
transparency  on  glass  or  celluloid  film. 

Transparencies. — Transparencies  are  made  by  the  regu- 
lar photographic  processes  of  exposure  and  development,  on 
glass  plates  or  films  placed  in  contact  with  the  negative,  or 
in  the  appropriate  position  in  an  enlarging  camera.  The 
sensitometry  and  the  terms  used  to  describe  the  qualities 
of  plate  or  film  for  this  purpose  are  those  already  given  in 
connection  with  the  general  discussion  of  plates  and  films. 
But  the  kind  of  emulsion  to  be  selected  is  quite  different 
from  the  aerial  negative  emulsion.  There  is  here  no  practical 
limitation  to  the  speed,  contrast  or  latitude.  Consequently, 
we  can  choose  a  positive  emulsion  on  which  the  exposure 
through  the  aerial  negative  falls  entirely  on  the  straight  line 
portion  of  the  characteristic  curve,  thus  reproducing  all  of 
its  tones,  and  the  contrast  of  the  negative  may  be  increased 
to  any  desired  extent.  The  possibilities  of  positive  emulsion 
are  indeed  rather  greater  than  the  usual  aerial  negative  can 
utilize.  A  range  of  clearly  graduated  opacities  of  two  or 
252 


PRINTING  MEDIA  253 

three  hundred  to  one  is  possible,  so  that  not  only  can  detail 
be  well  rendered  in  the  high-lights,  but  also  equally  well  in 
dark  shadows  where,  indeed,  an  increase  of  illumination  is 
necessary  for  it  to  be  made  easy  to  examine.  This  range  is 
to  be  contrasted  with  the  l-to-7  range  in  the  aerial  landscape, 
which  may  be  doubled  by  a  contrasty  plate.  In  resolving 
power,  the  positive  emulsion,  which  is  slow,  exceeds  the 
negative  emulsion.  It  easily  bears  examination  through  a 
magnifying  glass,  thus  making  any  enlargement  unnecessary 
in  the  printing  process. 

Glass  transparencies  are  of  course  impractical  for  general 
distribution,  on  account  of  their  fragility.  Heavy  film  trans- 
parencies are  not  open  to  this  objection,  and,  especially  in 
the  form  of  stereos,  constitute  the  most  beautiful  fcrm  cf 
aerial  photographic  print. 

Paper  Prints. — Prints  on  paper  suffer  by  compariscn 
with  transparencies,  in  the  range  of  tones  which  they  exhibit. 
This  lies  between  the  white  of  the  paper,  which  never  has 
more  than  80  per  cent,  reflecting  power,  and  its  darkest 
black,  which  differs  with  the  kind  of  paper.  In  dull  or  mat 
papers  the  blacks  will  reflect  as  much  as  5  per  cent. ;  in  glossy 
papers,  ordinarily  used  for  aerial  negatives,  the  reflection 
from  the  black  may  be  as  low  as  one  per  cent.,  but  in  order 
to  get  the  benefit  of  this  the  paper  must  be  so  held  as  not  to 
reflect  any  bright  object  to  the  eyes.  This  deficiency  in  the 
range  of  paper  gradations  is  not  so  serious  with  aerial  nega- 
tives as  with  ordinary  properly  exposed  negatives  because 
of  the  small  range  of  brightness  in  the  aerial  view. 

The  sensitometry  of  papers  is  similar  to  that  of  plates,  with 
the  difference  that  reflecting  powers  take  the  place  of  trans- 
parency. As  in  the  case  of  transparency  emulsions  there  is 
in  papers  no  dominating  requirement  for  extreme  speed,  to 
which  other  characteristics  must  be  subordinated.  Yet 


£54 


AIRPLANE  PHOTOGRAPHY 


speed  is  of  sufficent  importance  in  handling  large  quantities 
of  prints  so  that  serial  negative  printing  for  military  purposes 
has  been  done  almost  entirely  on  the  rapid  enlarging  papers, 
rather  than  on  the  true  contact  printing  papers,  which 
are  slower. 

The  two  principal  types  of  rapid  enlarging  papers,  the 


FIG.  110. — Characteristic  curves  of  bromide  paper. 


bromide  and  the  fc<gas  light,"  exhibit  certain  characteristic 
differences  which  are  important  to  bear  in  mind  in  seeking 
to  obtain  any  particular  quality  of  print.  Bromide  papers, 
of  which  "Nikko"  is  a  good  example,  show  sensitometric 
curves  rather  like  those  of  plates.  That  is,  they  increase  in 
contrast  with  continued  development.  At  the  same  time, 
as  is  shown  in  Fig.  110,  they  increase  somewhat  in  speed 


PRINTING  MEDIA 


255 


with  development;  that  is,  under  exposure  can  be  compen- 
sated for  to  a  small  degree  by  protracted  development. 
These  characteristics  of  bromide  paper  can  be  utilized  to 
secure  prints  of  a  quality  quite  different  from  that  of  the 
negative.  Thus,  if  the  negative  has  a  long  range  of  tones,  a 


5«pere.          1 
Erj-osuje 
TU»»  Be*. 
30  **e. 


FIG.  111. — Characteristic  curves  of  gas-light  paper. 


flat  print  can  be  secured  by  full  exposure  and  short  develop- 
ment. If,  as  is  apt  to  be  the  case  with  aerial  negatives,  a 
print  of  greater  contrast  than  the  negative  is  desired,  a  short 
exposure  with  lojig  development  is  called  for. 

The  sensitometric  curves  of  a  typical  gas  light  paper 
"Contrast  Enlarging  Cyco,"  are  shown  in  Fig.  111.  Here 
the  contrast  is  a  fixed  characteristic  of  the  paper,  and  the 


256        AIRPLANE  PHOTOGRAPHY 

only  effect  of  changing  development  is  on  the  speed;  that  is, 
exposure  and  development  are,  within  limits,  interchangeable. 
Choosing  a  printing  paper  is  a  matter  of  deciding  on  the 
contrast  required  for  the  class  of  negative,  and  selecting  a 
paper  which  will  give  this  contrast  with  a  good  range  of 
tones  from  a  clear  white  to  a  deep  black.  The  ideal  paper 
would  be  one  which  was  all  straight  line  in  the  H  &  D  plot. 
In  such  a  paper  there  would  occur  no  loss  of  contrast  in  the 
lighter  tones  when  the  high-lights  were  rendered  by  the  clear 
white  of  the  paper.  Too  great  contrast  with  a  short  straight 
line  portion,  results  in  loss  of  detail  at  the  ends  of  the  scale. 
A  negative  possessing  a  very  great  range  of  tones  cannot  be 
correctly  represented  on  one  paper  print — two  printings 
are  required,  one  for  high-lights  and  one  for  shadows,  but 
this  difficulty  is  rarely  to  be  faced  in  aerial  views.  The 
greatest  demand  for  aerial  printing  papers  has  been  for  those 
of  considerable  contrast,  because  of  the  flat  character  of 
the  negatives. 


CHAPTER  XXII 
PHOTOGRAPHIC  CHEMICALS 

General  Considerations. — Developing,  fixing  and  other 
chemicals  for  aerial  work  differ  in  no  essential  respect  from 
those  used  in  ordinary  photography.  Full  discussions  of 
these  are  to  be  found  in  numerous  texts  and  articles.  The 
aerial  photographic  problem  is  to  select  those  most  suited 
for  the  under-exposed  flat  negatives  characteristic  of  photo- 
graphs from  the  air.  At  the  same  time  selection  from  among 
the  chemicals  of  appropriate  quality  must  be  governed  by 
considerations  of  the  conditions  surrounding  work  in  aerial 
photographic  laboratories.  These  laboratories,  especially 
in  war-time,  are  apt  to  be  most  primitive  in  their  facilities. 

Characteristics  of  Developers  for  Plates  and  Films. — 
From  the  standpoint  of  practicability,  aerial  negative  devel- 
opers should  have  good  keeping  power,  be  slow  to  exhaust, 
and  work  well  over  a  considerable  range  of  temperatures. 
From  the  standpoint  of  the  photographic  quality  desired  in 
the  negative,  the  developer  should  bring  up  the  maximum 
amount  of  under-exposed  detail.  This  means  that  it  should 
impart  the  highest  possible  speed  to  the  plate,  with  good 
contrast,  and  low  fog  or  general  reduction  of  unexposed 
silver  bromide. 

There  are  many  characteristics  to  study  in  a  developer: 
its  effect  on  inertia  or  speed,  gamma  infinity,  fog,  time  of 
appearance,  "Watkins  factor,"  speed  of  development,  tem- 
perature coefficient,  dilution  coefficient,  keeping  power, 
exhaustion,  length  of  rinsing,  stain,  color  coefficient  and 
resolving  power.  These  are  defined  and  described  as  follows : 

Effect  on  inertia.  The  meaning  of  inertia  has  already 
17  257 


258        AIRPLANE  PHOTOGRAPHY 

been  given  under  the  discussion  of  plate  speed.  While  this 
is  a  constant,  independent  of  time  of  development,  for  any 
one  developer,  it  is  altered  appreciably  by  change  of  thelatter. 

Time-gamma  relation.  Contrast,  symbolized  by  7,  has 
likewise  been  discussed  under  plate  sensitometry.  Viewed 
from  the  standpoint  of  the  developer,  the  point  of  interest 
is -the  rate  at  which  7  varies  with  development,  and  the 
maximum  contrast  which  can  be  reached  or  7  infinity. 
Speed  of  development  is  commonly  defined  by  the  velocity 
constant,  symbolized  by  K,  which  is  arrived  at  mathemati- 
cally from  a  consideration  of  the  time  of  development  to 
produce  two  different  contrast  values.  High  7  infinity  is 
desired  for  aerial  negatives,  and  for  rapid  work  K  must 
also  be  high. 

Fog.  The  opacity  due  to  chemical  fog  is  to  be  kept  at  a 
minimum  in  aerial  negatives,  as  it  is  chiefly  prejudicial  to 
under  exposures. 

Time  of  appearance  and  Watkins  factor.  The  time  of 
appearance  is  measured  in  seconds.  The  Watkins  factor  is 
a  practical  measure  of  the  speed  of  development,  and  is  deter- 
mined by  the  ratio  of  the  time  of  development  required  for 
a  definite  Contrast,  to  the  time  of  appearance.  It  is  useful 
also  as  a  guide  to  development  time. 

Temperature  coefficient.  This  is  the  factor  by  which  the 
time  of  development  at  normal  temperature  (20  Cent.)  must 
be  increased  or  decreased  in  order  to  obtain  the  same  quality 
negative,  for  a  change  of  seven  degrees  either  side  of  normal. 

Temperature  limits  are  the  temperatures  between  which 
development  can  be  carried  out  with  any  degree  of  control 
or  without  serious  damage  to  the  negative.  These  factors 
are  of  great  importance  where  climatic  or  seasonal  changes 
have  to  be  endured. 

Dilution  coefficient.     This  is.  the  factor  by   which  the 


PHOTOGRAPHIC   CHEMICALS     259 

development  time  is  increased  in  order  to  maintain  a  given 
quality  negative  in  different  dilutions  of  the  developer.  It 
is  useful  in  tank  development. 

Keeping  power.  The  keeping  power  of  a  developer, 
mixed  ready  for  use,  is  determined  by  its  ability  to  resist 
aerial  oxidation.  A  developer  of  poor  keeping  power,  which 
must  be  made  up  immediately  before  use,  causes  delay  and 
waste  of  time  whenever  emergency  work  has  to  be  done, 
whereas  a  developer  of  good  keeping  power  may  be  left  in 
its  tank  ready  for  instant  use. 

Exhaustion  of  a  developer  is  the  rate  at  which  it  becomes 
useless  for  developing,  due  both  to  aerial  oxidation  and  to  the 
using  up  of  its  reducing  power  by  the  work  done  in  develop- 
ing plates.  It  is  conveniently  measured  by  the  area  of  plate 
surface  developable  before  the  solution  must  be  renewed. 

Length  of  rinsing.  The  time  required  for  rinsing  between 
development  and  fixing  bath  plays  a  not  unimportant  part 
in  total  development  time.  Dichroic  fog  is  caused  with 
some  developers  if,  due  to  insufficient  rinsing,  any  of  the 
caustic  alkali  is  carried  over  to  the  fixing  bath.  Stains 
develop  also  if  the  fixing  bath  is  old,  or  if  light  falls  on  the 
unfixed  plate  while  any  developer  remains  in  the  film. 

Color  coefficient.  The  function  of  the  sulphite,  which 
forms  a  constituent  of  all  developing  solutions,  is  two-fold. 
It  acts  partly  as  a  preservative,  and  partly  to  prevent  the 
occurrence  of  a  yellow  color  in  the  deposit.  The  yellow  color, 
if  present,  increases  the  photographic  contrast.  This  phe- 
nomenon has  been  purposely  utilized,  particularly  in  the 
British  service,  to  give  "stain"  to  negatives  which  otherwise 
would  show  insufficient  printing  density.  The  color  index 
or  coefficient  of  a  negative  (with  a  given  printing  medium) 
is  the  ratio  of  photographic  to  visual  density.  If  we  take  a 
pyro  developer  containing  five  parts  of  pyro  per  thousand 


260        AIRPLANE  PHOTOGRAPHY 

and  ten  parts  of  sodium  carbonate,  and  then  vary  the  amount 
of  sulphite  from  none  to  fifty  parts  per  thousand,  the  color 
index  varies  as  follows: 

Sulphite 
Parts  per  Thousand  Color  Index 

50  .16 

25  .24 

15  .30 

10  .45 

5  .80 

0  2.75 

The  color  index  is  somewhat  different  with  various  kinds 
of  printing  media. 

This  staining  effect  is  a  variable  one,  depending  upon 
length  of  development,  dilution  of  the  developer,  length  of 
rinsing,  temperature,  the  fixing  bath  used  (plain  hypo  being 
necessary  for  a  maximum  effect),  the  length  of  washing  after 
fixation  and  the  properties  of  the  water  used.  Standardiza- 
tion of  these  conditions  in  the  field  is  difficult;  hence  any 
developer  which  will  give  the  same  effective  contrast  without 
resorting  to  stain  is  to  be  preferred. 

Resolving  power.  Some  developing  processes  and  condi- 
tions will  introduce  bad  grain  into  the  negative.  Hence  the 
resolving  power  which  a  developer  brings  up  must  be  investi- 
gated among  its  other  characteristics. 

Practical  Developers  for  Aerial  Negatives. — In  the  English 
service  a  pyro  metol  developer  was  generally  used,  producing 
stained  negatives.  The  French,  American  and  Italian  prac- 
tice was  to  use  metol-hydrochinon,  without  staining.  A 
special  chlor-hydrochinon  developer,  worked  out  by  the 
Eastman  Research  Laboratory  for  the  United  States  Air 
Service,  has  probably  the  greatest  merit  of  any  yet  tried. 
A  comparison,  given  below,  between  it  and  a  pyro  metol 
formula  used  on  a  representative  plate,  illustrates  the  use 
of  the  various  bases  of  study  given  above. 


PHOTOGRAPHIC  CHEMICALS     261 

Solution  A  PYRO  FORMULA  Solution  B 

Pyro,  3.75  grams  Sodium  carbonate,  53  g 

Potassium  metabisulphite,  3.75  g 
Metol,  3.05  g 
Potassium  bromide,  1.5  g 
Water,  500  c.c.  Water,  500  c.c. 

Use  1  part  of  A  to  1  of  B 
CHLORHYDROCHINON  FORMULA 

Solution  A  Solution  B 

Chlorhydrochinon,  25  g  Sodium  carbonate,  30  g 

Metol,  6  g  Sodium  hydrate,  10  g 

Sodium  bisulphite,  2.5  g  Potassium  bromide,  3  g 
Sodium  sulphite,  25  g 

Water  to  670  c.c.  Water  to  330  c.c. 

Use  2  parts  of  A  to  1  of  B 

Pyro      Chlorhydrochinon 

H  &  D  speed  150  180 

Gamma  infinity  1.45  2.12 

Fog  (at  maximum  gamma)  .32  .60 

Time  of  appearance  5  seconds    5  seconds 

Watkins  factor  25  10 

Velocity  factor  "K  "  .320  .400 

Temperature  coefficient  1.40  2.0 

Temperature  limits  4°  to  32°  C  4°  to  32°  C 

Keeping  power  45  minutes     8  days 

Exhaustion  (100  c.c.)  30  sq.  in.     300  sq.  inches 
Dilution  coefficient  2  2 

Color  coefficient  1.50  1.00 

Resolving  power  47  53 

Owing  to  the  difficulty  of  securing  pure  Chlorhydrochinon 
a  metol  hydrochinon  of  very  similar  properties  has  been 
worked  out.  Its  composition  is 

Metol  16  grams 

Hydrochinon  16     ' 

Sodium  sulphite  .      60     ' 
Sodium  hydroxide  10 

Potassium  bromide  10     " 

Water  to  1  litre 

To  keep  the  ingredients  in  solution  in  cold  weather,  50 
c.c.  of  alcohol  should  be  included  in  every  litre  of  solution. 
All  things  considered  this  is  probably  the  most  practical  and 
satisfactory  developer  for  aerial  negatives. 


AIRPLANE  PHOTOGRAPHY 

Developers  for  Papers. — The  following  formula  has  been 
found  very  satisfactory  for  papers : 

Metol  .9  gram 

Hydrochinon  3.6 

Sodium  carbonate  20.0 

Sodium  sulphite  14.0 

Potassium  bromide  .5  to  1.0      " 
Water  to  1    litre 

Fixing  Baths. — For  plates  the  following  fixing  and  hard- 
ening bath  is  recommended : 

Sodium  thiosulphate  (hypo)  350  grams 

Potassium  chrome  alum  6 

Sodium  bisulphite  10     '  * 

Water  to  1000  c.c. 

During  hot  weather,  the  above  quantities  of  chrome 
alum  and  bisulphite  are  doubled. 
For  papers  the  following: 

Hypo,  35  per  cent.         100  volumes 
Acid  hardener  5         " 

The  acid  hardener  is  constituted  as  follows: 

Alum  50  grams 

Acid  acetic  28°  400  c.c. 

Sodium  sulphite  100  grams 
Water  to  1  litre 

Intensification  and  Reduction. — These  processes  have 
been  little  employed  in  air  work.  Reduction  is  rarely  neces- 
sary, for  obvious  reasons.  Intensification  would  often  be  of 
value,  but  the  common  practice,  which  saves  some  time,  is 
to  use  printing  paper  of  strong  contrast  for  those  negatives 
which  are  deficient  in  density  and  contrast.  When  intensi- 
fication is  desirable  or  permissible,  either  the  ordinary  mer- 
cury or  uranium  intensifier  may  be  used. 

Water. — In  the  field  it  is  found  necessary  in  many  cases 
to  purify  the  water  that  is  to  be  used  in  mixing  up  chemicals. 


PHOTO GRAPHIC   CHEMICALS     263 

Water  may  contain  suspended  matter  or  dirt,  dissolved 
salts,  and  slime.  It  is  important  to  remove  the  suspended 
matter,  as  it  may  cause  spots  on  the  plates  and  papers, 
while  any  slime  would  coagulate,  forming  a  sludge  in  the 
developer  which  would  also  tend  to  settle  on  the  plates  and 
cause  marks  during  development.  The  dissolved  salts  may 
or  may  not  cause  trouble.  Two  methods  of  purification 
are  possible: 

(a)  Filter  the  water  through  a  cloth  into  a  barrel,  add 
about  one  gram  of  alum  for  every  four  litres  of  water,  and 
allow  to  settle  over  night.    Draw  off  the  clear  liquid  from  a 
plug  in  the  side  as  required. 

(b)  Boil  the  water  and  allow  it  to  cool  over  night.    If  the 
water  contains  dissolved  lime,  boiling  will  often  cause  this 
to  come  out  of  solution. 


.V 

METHODS  OF  HANDLING  PLATES,  FILMS 
AND  PAPERS 


CHAPTER  XXIII 

THE    DEVELOPING    AND    DRYING    OF    PLATES 

AND   FILMS 

Field  Requirements. — Developing,  fixing,  drying  and 
printing  in  the  field  demand  simple  and  convenient  apparatus 
that  may  be  carried  about  and  installed  with  the  least 
amount  of  labor.  On  top  of  these  requirements  military 
needs  impose  others  that  are  more  difficult.  Speed  is,  on 
occasion,  imperative.  A  print  may  be  required  within  a 
few  minutes  after  landing,  and  many  thousands  within  a 
few  hours.  Quantity  production  must  be  achieved  under 
the  most  primitive  conditions.  Nothing,  in  fact,  shows  the 
calibre  of  the  photographic  officer  better  than  his  choice  of 
workplaces  as  the  army  moves  forward.  Ingenuity  and 
practical  judgment  are  at  a  premium.  Cellars,  stables,  dog 
kennels,  or  huts  hastily  built  from  packing  cases,  must  be 
equipped  and  in  working  order  over  night.  All  the  facilities 
offered  by  a  great  city  are  urgently  needed — water,  electric 
light,  power  for  driving  fans — but  must  be  dispensed  with 
if  the  photographic  section  is  to  be  convenient  to  the  air- 
drome, whose  portable  hangars  are  most  apt  to  be  pitched 
in  the*  open  country.  Water  must  be  carried,  electricity 
generated,  and  to  the  photographic  problem  is  added  the 
military  one  of  concealment  and  protection.  Dugouts  and 
bomb  proofs  must  be  built  for  supplies,  and  "funk  holes" 
for  the  men.  Entire  underground  emergency  extensions 
have  sometimes  been  built  in  stations  occupied  for  extended 
periods,  for  airdromes  are  a  favorite  bombing  target. 

For  getting  the  exposed  plates  to  the  photo  section, 
messengers,  on  motorcycles  if  possible,  are  employed.  In 

267 


268 


AIRPLANE  PHOTOGRAPHY 


some  cases,  where  hangars  and  photographic  hut  are  forced 
to  be  widely  separated,  recourse  has  been  had  to  parachutes 
(Fig.  112),  a  device  also  employed  to  distribute  prints  to 
infantry  during  an  advance. 

For  warfare  of  movement,  especially  in  sparsely  settled 


FIG.  112. — Receiving  pictures  from  plane  by  parachute. 

or  devastated  country,  where  cellars  are  unavailable,  the 
dark  room  must  be  taken  along.  Motor  trucks  and  trailers 
(Figs.  113,  114,  115),  the  former  for  hauling  supplies  and 
electric  light  generating  plant,  the  latter  fitted  as  a  complete 
developing  and  printing  laboratory,  form  the  headquarters 
of  each  photographic  section  in  the  field.  Usually  altogether 
too  small  for  the  amount  of  work  required,  they  were  ex- 


PLATES  AND  FILMS 


tended  by  tents  and  lean-to's,  or  ingeniously  used  as  a 
nucleus  for  the  organization  of  the  favored  stable  or  cellar. 
Methods  of  Plate  Development. — Where  speed  is  not 
required  the  simplest  and  commonest  mode  of  developing 
plates  is  in  the  tray,  one  plate  at  a  time.  Common  practice 
is  to  examine  the  plate  at  intervals  during  development, 
and  discontinue  the  operation  on  the  basis  of  its  appearance. 


FIG.  113. — Mobile  photographic  laboratory. 

This  is  only  possible  if  the  plates  used  are  insensitive  to  some 
light  by  which  the  eye  can  see.  Deep  red  light  is  suitable 
for  ordinary  and  most  orthochromatic  plates.  A  faint  blue- 
green  may  be  used  with  some  panchromatic  plates.  The 
best  practice,  however,  is  to  develop  by  time  in  total  darkness, 
whereby  all  chance  of  dark  room  fog  is  avoided.  Develop- 
ment time  for  plates  of  the  average  exposure  of  the  one  to 
be  developed  is  either  known  from  previous  experience,  or  is 
found  by  trial  on  the  first  one.  Development  by  time 


270 


AIRPLANE  PHOTOGRAPHY 


results  in  negatives  of  densities  varying  with  the  exposures, 
but,  as  was  brought  out  in  the  discussion  of  sensitcmetry, 
this  difference  can  be  compensated  for  by  the  choice  of  the 
paper  used  for  printing,  and  by  its  treatment. 


FIG.  114. — Interior  of  photographic  trailer,  developing  room. 

Where  larger  quantities  of  plates  are  to  be  handled  tank 
development  is  adopted.  In  ordinary  tank  development  the 
plates  are  placed  in  grooved  tanks,  into  which  is  poured  first 
the  developer,  next  the  rinsing  water,  and  then  the  hypo. 
It  has  been  customary  in  tank  development  as  practiced  for 
peace-time  work  to  use  dilute  developer,  requiring  from  ten 


PLATES  AND  FILMS 


271 


to  thirty  minutes,  but  speed  requirements  in  war-time  aerial 
photography  dictate  the  use  of  full-strength  quick-acting 
developer.  An  improvement  on  the  simple  grooved  tank  is 
provided  by  metal  cages  or  racks,  each  holding  a  dozen  or 


FIG.  115. — Interior  of  photographic  trailer.    Enlarging  camera  and  printer. 


more  plates,  which  may  be  introduced  or  removed  from  the 
tank  as  a  Unit  (Fig.  116). 

The  core  rack  system  combines  certain  of  the  features  of 
both  tray  and  tank  development.  Each  plate  is"  inserted -in 
^a  separate  utetal  frame  with  project  ing -lugs  to-  rest  on  the 


AIRPLANE  PHOTOGRAPHY 


top  of  the  tank  and  so  suspend  the  plate  in  the  solution. 
The  process  of  development  is  the  same  as  in  the  tank  system, 
but  any  individual  plate  may  be  examined  and  removed. 

Film  Developing  and  Fixing. — The  problem  of  quicky 
handling  roll  film  of  large  size  is  one  upon  whose  solution 
depends  in  large  degree  the  feasibility  of  film  cameras  for 


Via.  116. — Tank  and  rack  for  tank  development. 

aerial  work.  It  presents  many  difficulties:  a  long  film  is 
unwieldy,  is  inherently  subject  to  curling,  and  takes  up  much 
space  if  it  is  handled  entire.  For  small  scale  operations  roll 
film  can  be  cut  into  short  strips  and  developed  either  by 
drawing  through  a  tray  or,  if  cost  of  developer  is  no  object, 
in  a  deep  tank.  In  order  to  make  the  cutting  apart  of  expos- 
ures easy  in  the  dark,  film  cameras  should  make  some  form 
of  punch  mark  in  the  film  between  the  exposed  parts,  or  the 
space  between  exposures  should  be  uniform,  so  that  a  print 


PLATES  AND  FILMS  273 

trimmer  set  to  a  definite  mark  may  be  used.  Racks  for  hold- 
ing two  or  three  feet  of  film,  folded  back  on  itself  and  clasped 
by  spring  clothes-pins,  are  fairly  practical.  One  object  of 
the  use  of  film,  however,  is  to  greatly  increase  the  number  of 
possible  exposures;  and  where  hundreds  instead  of  dozens 
of  exposures  are  to  be  developed,  this  method  takes  up 
entirely  too  much  time. 

Following  the  practice  in  moving  picture  development, 
film  developing  machines  of  various  designs  have  been  devised. 
Among  these  may  be  described  the  G.  E.  M.  machine;  the 
Ansco  machine;  the  Eastman  apron  machine;  the  Brock 
frame  and  tank  apparatus;  the  Eastman  reel  machine;  and 
a  modification  of  the  latter  by  the  United  States  Air  Service. 

The  G.  E.  M.  film  developing  apparatus,  similar  in  idea 
to  the  Eastman  "apron"  method  of  film  developing,  as  exem- 
plified in  the  familiar  amateur  film  developing  machines,  has 
the  film  wound  in  a  spiral  on  a  long  linked  metal  frame  or 
chain.  After  being  wound  it  is  placed  in  a  tub  of  developer, 
from  that  to  a  tub  of  water,  thence  to  a  tub  of  hypo,  and 
finally  to  a  tub  of  water,  where  it  is  washed  in  several  changes. 
The  objections  to  the  method  are  that  it  takes  up  much 
floor  space  for  the  various  tubs,  and  that  it  requires  such 
large  quantities  of  solution.  To  develop  a  thirty-five  foot 
length  of  18  X24  centimeter  exposures  requires  approximately 
28  gallons  of  developer;  for  the  rinsing,  28  gallons  of  water, 
and  the  same  for  hypo,  and  at  least  three  times  that  for 
washing.  In  all  168  gallons  of  water  must  be  brought  to  the 
developing  hut  or  lorry. 

The  Ansco  machine  makes  use  of  an  idea  frequently 
applied  in  the  moving  picture  industry.  The  film  is  carried 
spirally,  upon  two  cross-arms  which  bisect  each  other  at 
right  angles,  and  which  contain  vertical  pins  around  which 
the  film  is  looped,  beginning  at  the  center  and  working  out. 
'is 


274        AIRPLANE  PHOTOGRAPHY 

After  it  is  wound  it  is  placed  in  a  tub  of  developer,  as  in  the 
G.  E.  M.  machine.  It  has  an  advantage  over  this  apparatus 
in  that  the  shape  of  the  tubs  or  tanks  is  square  instead  of 
round.  But  it  is  equally  extravagant  of  space  and  water. 

This  same  criticism  may  be  made  of  the  Eastman  apron 
apparatus  for  film  developing.  This  is  similar  to  the  G.  E.  M. 
machine,  but  differs  from  it  in  using  a  perforated  celluloid 
apron  to  support  the  film  during  the  various  operations, 
instead  of  a  metal  chain. 

The  Brock  developing  outfit  consists  of  a  rectangular 
wooden  frame  and  a  three-compartment  tank.  The  frame, 
which  is  approximately  3  by  4  feet  in  size,  is  used  as  a  support 
for  the  4  inch  wide  film,  which  is  wound  spirally  around  it, 
between  guiding  pins.  ^A  special  support  is  provided,  on 
which  the  frame  may  be  rotated  as  the  film  is  fed  off  the 
camera  spool.  The  frame,  with  the  film  on  it,  is  lowered 
successively  into  the  three  narrow  but  deep  compartments 
of  the  developing  tank.  The  first  compartment  holds 
developer,  the  next  water,  the  next  hypo.  The  amount  of 
developing  solution  required  is  rather  large  (96  gallons  of 
water  in  all  for  a  strip  of  100  4X5  inch  exposures),  but 
because  of  the  small  surface  exposed  to  the  air,  it  keeps  for 
a  considerable  period.  The  chief  demand  for  floor  space  with 
this  apparatus  is  for  feeding  the  film  on  to  the  frame. 

In  the  Eastman  twin  reel  machine  the  film  is  wound  on  a 
wooden  drum  or  reel  of  large  diameter,  to  form  a  helix.  The 
drum  is  suspended  so  that  the  bottom  edge  touches  the  devel- 
oping solution,  and,  upon  revolving  the  drum,  every  portion 
of  the  helix  of  film  is  brought  into  contact  with  the  developer. 
By  shaping  the  developing  tank  so  that  it  closely  conforms 
to  the  shape  of  the  reels,  a  high  economy  in  quantity  of 
developing  agent  can  be  achieved.  When  developing  action 
is  finished,  the  developer  is  emptied  out,  rinse  water  put  in; 


PLATES  AND  FILMS  275 

hypo  follows,  and  then  comes  the  final  washing  with  water. 
With  this  apparatus  the  whole  cycle  is  completed,  for  the  35 
feet  length  of  film  above  considered,  with  s.evengallonsof  water. 

The  Air  Service  apparatus  differs  from  the  above  only 
in  the  drying  method,  which  will  be  described  below. 

Heavy  cut  film,  such  as  is  marketed  under  the  name  of 
Portrait  Film,  has  not  thus  far  been  used  in  aerial  work, 
except  for  printing  transparencies.  It  is  conceivable,  however, 
that  film  in  the  cut  form  may  be  used  in  some  future  design 
of  camera.  This  may  be  developed  expeditiously  in  a  tray, 
six  or  eight  films  being  handled  at  a  time,  in  a  pile,  pulling 
out  the  lower  one  frequently  and  placing  it  on  top.  The 
core  rack  system  is  also  available  for  film  in  this  form,  special 
racks  with  clips  to  hold  the  film  being  necessary. 

Plate  Drying. — The  drying  of  negatives  on  glass  is  a 
comparatively  simple  matter,  owing  to  the  rigid  nature  of 
the  emulsion  support.  A  large  number  of  plates  may  be 
placed  in  a  compact  mass  in  the  ordinary  plate  racks  of 
commerce  with  the  wet  sides  accessible  to  a  draft  of  air. 
Two  dozen  plates  separated  from  each  other  by  a  quarter 
of  an  inch  and  left  to  dry  spontaneously  in  a  room  of  ordi- 
nary humidity  and  living  temperature  will  dry  in  two  hours 
and  a  half.  If  the  surface  be  wiped  with  soft  cheese-cloth 
or  chamois,  so  as  to  absorb  all  the  surface  moisture  before 
the  plates  are  placed  on  the  rack,  this  time  may  be  appreci- 
ably reduced.  By  placing  the  plates  in  a  forced  draft  of  air, 
from  an  electric  fan,  this  time  may  be  reduced  to  an  hour. 

Extra  rapid  drying  of  plates  may  be  accomplished  by 
placing  them  in  a  bath  of  alcohol  before  putting  them  in  the 
racks.  The  alcohol  displaces  all  the  water  in  the  film,  and 
is  itself  very  quickly  dissipated  into  the  atmosphere  when  the 
plate  is  taken  from  the  tray.  The  plate  must  be  left  in  the 
alcohol  tray  long  enough  for  the  substitution  of  the  alcohol 


276        AIRPLANE  PHOTOGRAPHY 

for  the  water  in  the  film  to  take  place.  Five  minutes  is  long 
enough.  The  alcohol  before  use  must  be  as  nearly  free  from 
water  as  possible.  The  best  way  to  make  sure  of  this  is  to 
place  in  the  bottle  of  alcohol  some  lumps  of  calcium  oxide, 
which  will  take  up  the  water  and  form  calcium  hydroxide, 
which  settles  at  the  bottom  of  the  bottle. 

Another  method  of  quick  plate  drying  takes  advantage 
of  the  extraordinary  greediness  of  potassium  carbonate  for 
water.  The  wet  plates  are  placed  in  a  saturated  solution  of 
potassium  carbonate  and  left  for  a  minute.  If  a  plate  be  now 
taken  from  the  solution  and  its  surface  wiped  with  a  soft 
cloth,  it  will  be  found  that  the  film  has  a  greasy,  slippery 
feeling,  but  that  it  contains  no  water  and  can  be  printed  from 
at  once.  Plates  so  treated  should  be  washed,  however,  at 
some  time  in  the  succeeding  four  months,  or  the  traces  of 
potassium  carbonate  left  in  the  film  cause  deterioration. 

Film  Drying. — Unlike  the  drying  of  plates,  drying  of  film 
negatives  is  a  very  puzzling  problem,  and  may  be  considered 
as  the  crux  of  the  successful  use  of  film  in  aerial  cameras. 

Apron  and  similar  machines  have  very  poor  drying  effi- 
ciency if  the  film  is  left  in  place,  for  not  only  the  film  but  the 
apron  or  chain  must  be  freed  of  water.  This  may  be  has- 
tened, as  in  the  G.  E.  M.  machine,  by  blowing  air  through 
with  fans,  but  even  with  their  help  drying  a  35  foot  film  is  a 
matter  of  two  hours  or  more.  Passing  the  film  through 
wringers  or  a  squeegee  to  remove  excess  water  is  a  consider- 
able aid ;  the  film  may  either  be  re-wound  on  a  dry  reel,  to  be 
put  in  a  forced  draft  of  air,  or  may  be  hung  up  in  short  lengths 
or  festooned,  either  method  taking  up  a  great  deal  of  space. 
The  use  of  alcohol  is  not  advisable  as  it  may  abstract  cam- 
phor from  the  celluloid  and  cause  the  film  to  become  distorted. 

The  Eastman  twin  reel  machine  had  an  upper  reel  joined 
to  the  lower  or  developing  reel,  with  a  chain  and  sprockets, 


PLATES  AND  FILMS 


277 


so  that  the  upper  reel  revolved  at  the  same  time  and  rate  of 
revolution  as  the  lower,  when  the  lower  was  being  revolved 
at  the  gentle  speed  appropriate  to  the  developing  process. 
Fans  blew  a  draft  of  air  over  the  upper  reel.  This  method 
necessitated  over  an  hour  for  drying. 


FIG.   117. — U.  S.  Air  Service  film  developing  machine  for  film  24  centimeters  wide. 

The  Air  Service  model  of  film  developing  and  drying 
machine  (Fig.  117)  introduces  an  essential  modification  in 
the  drying  scheme  of  the  Eastman  apparatus.  The  upper 
reel  is  quite  independent  of  the  lower  reel  and  is  revolved  at 
a  high  rate  of  speed,  so  that  a  whirling  action  is  introduced 
into  the  drying.  Large  rotating  fans  at  the  same  time  drive 


278        AIRPLANE  PHOTOGRAPHY 

a  considerable  volume  of  air  across  the  film  surface,  and  the 
combination  of  the  two  agencies  makes  it  possible  to  dry 
35  feet  of  18  X24  centimeter  film  in  20  to  30  minutes.  This 
for  large  numbers  of  pictures  makes  the  use  of  film  even 
quicker  than  that  of  plates.  The  only  practical  drawback 
to  the  apparatus  is  its  bulk,  which  calls  for  a  separate  room 
or  trailer.  This,  however,  seems  to  be  inevitable  in  the  use 
of  large  roll  film. 

Cut  film  can  be  dried  with  speed  only  if  placed  in  a  draft 
of  warm  air.  Drying  boxes,  with  a  chute  or  chimney  and 
with  fans  to  drive  the  air  through  from  an  alcohol  stove, 
will  dry  several  dozen  films  in  an  hour.  The  films  must  not 
be  closer  together  than  about  one  inch,  which  makes  the 
drying  boxes  rather  cumbersome. 

Marking  Negatives. — After  development  and  drying, 
and  before  filing  or  printing,  each  plate  should  be  marked 
with  data  for  purposes  of  future  identification.  This  is  most 
easily  done  with  pen  and  ink  on  the  film  side  (in  reversed 
lettering)  either  along  an  edge  in  the  unexposed  portion 
covered  by  the  sheath  or  in  a  corner,  so  as  to  lose  as  little 
of  the  photograph  as  possible.  Just  what  data  shall  be  in- 
scribed is  dictated  by  the  purpose  for  which  the  negative  was 
made.  The  date,  altitude,  time  of  day,  true  north  (from 
known  permanent  features  or  from  shadow  direction  and 
time  of  day),  number  of  the  camera  used,  the  focal  length  of 
the  lens.  Other  records,  such  as  the  plane  and  squadron 
numbers,  or  even  the  pilot's  and  observer's  initials,  may  be 
called  for  (Fig.  75).  For  mapping  work  the  scale  of  each 
of  a  set  of  negatives,  once  found,  may  be  marked,  either  in 
figures  or  by  means  of  a  line  of  length  corresponding  to  a 
fixed  distance  on  the  ground.  Rectifying  data  can  similarly 
be  inscribed,  so  that  the  negative  can  be  printed  in  the  en- 
larging and  rectifying  camera  with  the  minimum  of  delay. 


CHAPTER  XXIV 
PRINTING  AND  ENLARGING 

Contact  Printing. — Single  prints  are  made  most  simply 
in  a  printing  frame  held  at  a  short  distance  from  a  light 
source.  When  any  quantity  must  be  made,  as  in  turning 
out  prints  at  high  speed  for  distribution  to  an  army  before 
an  attack,  printing  machines  are  employed.  These  consist 
essentially  of  a  light  box,  a  printing  frame  of  plate  glass, 
and  a  pressure  pad.  In  the  commercial  models,  such  as  the 
Crown  and  the  Ansco,  which  are  equipped  with  electric  light, 
merely  bringing  the  pressure  pad  down  and  clamping  it 
automatically  turns  on  the  light,  while  release  of  pressure 
terminates  the  exposure. 

The  question  of  regulating  the  distribution  of  light  is  of 
considerable  importance  with  negatives  taken  by  focal-plane 
shutters  of  non-uniform  rate  of  travel.  In  the  Mclntire 
printer  (Fig.  119),  the  separate  electric  bulbs  are  on  long 
necks  in  ball  and  socket  joints,  so  that  they  can  be  brought 
individually  closer  to  the  printing  surface  or  farther  away 
from  it,  thus  permitting  a  wide  range  of  "dodging."  This 
printer  also  has  an  automatic  time  control  for  the  light,  a 
valuable  device  where  many  prints  from  the  same  negative 
are  desired. 

These  machines  are  well  suited  for  printing  aerial  nega- 
tives, either  plate  or  cut  film,  if  used  where  a  source  of  elec- 
tric current  is  available.  The  chief  defect,  which  may  be 
caused  by  faulty  construction,  is  imperfect  contact  between 
paper  and  negative,  a  cause  of  serious  unsharpness  on  prints 
destined  for  minute  study  in  interpretation. 

The  printing  of  aerial  negatives  may  be  done  either  on 

279 


280        AIRPLANE  PHOTOGRAPHY 


FIG.  118. — Printing  machine. 


roll  or  cut  paper,  and  if  films  are  used,  a  further  alternative 
is  offered  of  handling  it  either  in  the  roll  or  in  cut  form. 
Where  many  prints  are  to  be  made  from  one  negative  roll 


PRINTING  AND  ENLARGING      281 

paper  has  some  advantages,  particularly  if  a  developing  and 
drying  machine  is  available.  But  for  moderate  numbers  the 
advantage  is  small,  since  cut  prints  can  be  developed  quite 


FIG.  119. — Interior  of  Mclntire  printer,  showing  lamps  adjustable  in  position  for  "dodging." 

conveniently  in  goodly  numbers  in  the  ordinary  trays.  But 
the  advantages  of  keeping  film  in  the  roll  form  are  very 
great,  both  in  respect  to  storage  and  in  respect  to  handling 
during  printing,  as  the  rollers  provide  the  necessary  tension 
and  prevent  the  film  "getting  away." 


282 


AIRPLANE  PHOTOGRAPHY 


For  the  American  Air  Service,  cut  paper  has  been  used 
exclusively.    For  film  printing,  the  Ansco  machine  has  been 


FIG.  120. — Film  printing  machine. 


equipped  with  roll  pivots  to  take  film  24  centimeters  wide 
which  may  be  advanced  in  either  direction  by  turning  large 
milled  heads  (Fig.  120).  If  we  put  rollers  on  the  two  remain- 


PRINTING  AND  ENLARGIN'G      283 

ing  sides  of  the  box  to  handle  paper  we  transform  the  printer 
into  the  same  form  as  a  French  machine,  in  which  paper  and 
film  are  moved  at  right  angles  to  each  other.  A  disadvantage 
of  this  modification,  however,  is  the  difficulty  of  examining 
the  negative  to  be  printed. 

Stereo  Printing. — To  make  separate  prints  from  the  two 
elements  of  a  stereoscopic  pair  and  mount  them  side  by  side 
after  proper  orientation  is  too  slow  a  process  if  quantities 
of  prints  are  needed.  One  method  of  multiple  production 
is  to  make  a  master  stereogram,  and  then  produce  photo- 
graphic copies  of  it,  but  there  is  inevitable  loss  of  quality 
in  this  copying  process.  An  intermediate  method  is  to  print 
from  both  negatives  on  the  same  sheet  of  paper.  In  order 
to  do  this  the  negatives  must  be  placed  in  rather  large  frames, 
with  mats  properly  located  to  guide  the  placing  of  the  paper. 
The  Richard  double  printing  frame"  is  a  practical  device 
which  simplifies  the  necessary  manipulations.  It  consists 
essentially  of  a  platform  pierced  with  three  illuminated 
openings.  The  two  negatives  are  compared,  superposed, 
and  orientated  over  the  central  opening  and  then  shifted 
laterally,  one  to  each  of  the  two  side  openings,  which  serve 
both  as  printing  frames  and  masks.  The  printing  back 
slides  on  a  rod,  permitting  the  paper  to  be  lifted  up  and 
moved  between  exposures.  Once  the  negatives  are  properly 
placed,  stereo  prints  can  be  turned  out  quickly  and  easily. 

Enlarging. — In  the  French  service  contact  printing  was 
the  rule  during  the  war.  The  English  practice,  on  the  other 
hand,  was  to  take  small  negatives — 4X5  inches,  with  8  to 
12  inch  lenses — and  enlarge  them,  usually  to  6^X8^/2  inches. 
For  this  purpose  a  regular  part  of  the  English  photo  section 
equipment  was  the  enlarging  camera  (Fig.  115).  This  may 
be  briefly  described  as  a  short  focus  camera  in  which  the 
subject  to  be  photographed  is  a  negative,  illuminated  by 


284        AIRPLANE  PHOTOGRAPHY 

transmitted  light,  whose  image  is  thrown  by  the  camera 
lens  on  the  paper  or  other  sensitive  surface.  By  making 
the  distance  between  negative  and  lens  less  than  that 
between  lens  and  paper,  the  resulting  print  is  an  enlarge- 
ment, and  vice  versa.  The  scale  of  enlargement  or  of  reduc- 
tion is  varied  over  limits  set  only  by  the  length  of  the  camera 
and  the  amount  of  light  available. 

The  lens  employed  must  of  course  possess  sufficiently 
high  quality  to  preserve  all  the  sharpness  of  the  negative, 
and  focussing  must  be  done  with  accuracy.  Next  to  the  lens 
the  most  important  element  is  the  light  source.  This  may 
be  of  the  point  form,  such  as  a  concentrated  filament  electric 
lamp,  an  oxy-acetylene  lime  light,  or  an  acetylene  flame. 
The  latter  was  extensively  used  in  the  English  service,  while 
acetylene  generators  for  emergency  purposes  formed  part  of 
each  American  photo  truck  equipment.  With  point  light 
sources  we  must  use  condensers  to  focus  the  light  into  the 
projecting  lens.  Much  less  efficient,  but  the  only  recourse 
where  large  condensers  are  not  available,  is  a  diffusing  glass 
behind  the  negative,  illuminated  either  by  a  bank  of  electric 
lamps  with  mirrors  or  by  a  IT  tube  mercury  vapor  lamp, 
where  proper  current  can  be  got. 

The  device  for  holding  the  printing  paper  must  permit 
quick  changing,  but  insure  good  contact.  We  may  use 
either  a  spring  plate  to  hold  the  paper  against  plate  glass 
from  behind,  or  else  a  weight  acting  on  a  lever  arm  of 
sufficient  length. 

The  need  for  some  automatic  means  of  focussing  an 
enlarging  camera  has  been  very  generally  felt.  An  illustra- 
tion of  such  an  enlarging  camera  is  that  put  out  by  Williams, 
Brown  &  Earle,  of  Philadelphia,  known  as  the  "Semper- 
focal"  (Fig.  121).  In  this  camera  the  movements  of  the  lens, 
paper  easel  and  negative  are  so  inter-related  and  actuated 


PRINTING  AND  ENLARGING      285 

with  respect  to  each  other  that  the  correct  focus  of  the 
instrument  is  maintained  for  any  degree  of  enlargement 
or  reduction.  This  feature  is  a  great  help  in  making  up 
mosaic  maps,  where  prints  of  continuously  varying  scale 
ordinarily  occasion  serious  delay  for  individual  focussing. 

Determining  the  correct  enlargement  for  each  negative 
of  a  mosaic  is  perhaps  the  most  important  problem  in  the 
use  of  the  enlarging  camera  for  aerial  work.  The  correct 


FIG.    121. — "Semperfocal"   enlarging  camera,   with   mechanism   for   holding     image  in    focus    at 

any  enlargement. 

setting  of  the  camera  may  be  found  by  either  of  two  methods : 
the  negative  may  be  previously  scaled  and  marked  with  a 
line  on  its  edge,  which  must  be  projected  to  a  definite  size; 
or  the  true  location  of  several  points  in  the  picture  as 
obtained  from  an  accurate  map  may  be  marked  on  the 
enlarging  camera  easel  according  to  the  desired  scale,  and 
the  negative  image  projected  to  coincide  with  these.  In 
either  case,  if  an  exact  scale  is  desired,  allowance  must  be 
made  for  paper  shrinkage,  a  matter  which  must  be  deter- 
mined by  previous  experiment. 


AIRPLANE  PHOTOGRAPHY 

Rectifying. — Negatives  taken  when  the  plane  is  not 
flying  level  will  be  distorted  (Figs.  134  and  135).  Contact 
prints  from  these  will  not  fit  into  a  mosaic,  and  no  mere 
enlargement  or  reduction  wrill  make  them  available.  It  is 
necessary  with  these  negatives  to  resort  to  a  rectifying 
camera.  This  is  an  enlarging  camera  built  so  that  the  nega- 
tive and  print  easel  may  be  inclined  about  vertical  and 
horizontal  axes,  thereby  purposely  introducing  a  distortion 
sufficient  to  offset  the  distortion  of  the  negative.  Thus,  if 
the  bottom  of  the  printing  surface  is  moved  away  from  the 
lens,  that  part  of  the  picture  will  be  enlarged;  if  moved 
toward  the  lens,  reduced. 

For  small  rectifications  the  common  practice  is  to  tilt  the 
printing  surface  alone,  a  method  that  is  practical  as  long  as 
this  tilting  does  not  affect  the  focus  so  much  as  to  require 
prohibitive  stopping  down  of  the  lens.  For  great  distortions, 
such  as  that  inherent  in  the  principle  of  the  Bagley  camera, 
it  is  necessary  to  tilt  both  negative  and  print  in  order  to 
preserve  an  approximate  focus,  a  given  portion  of  the  nega- 
tive moving  toward  the  lens  as  the  corresponding  portion 
of  the  print  is  moved  away.  Both  schemes  for  rectification 
are  shown  diagrammatically  in  Fig.  122. 

Developing  and  Drying  Prints. — The  developing  of  prints 
follows  closely  that  of  cut  or  roll  film,  and  so  need  not  be 
treated  separately. 

The  drying  of  emulsions  on  paper  is  more  easily  accom- 
plished than  the  drying  of  emulsions  on  glass,  for  two 
reasons:  the  emulsions  on  paper  are  much  more  thinly 
coated,  and  there  is  diffusion  of  moisture  into  the  atmosphere 
from  front  and  back  of  the  printing  medium.  In  the  field 
a  common  method  has  been  to  soak  the  prints  in  water-free 
alcohol  and  then  burn  off  the  alcohol,  thus  securing  a  dry 
print  within  two  or  three  minutes  after  the  conclusion  of 


PRINTING  AND  ENLARGING      287 

washing.  A  later  method  very  generally  employed  is  to 
cover  wooden  frames  three  or  four  feet  above  the  ground 
with  chicken  wire^or  muslin,  and  on  these  lay  the  prints  after 
soaking  them  in  alcohol.  Below  the  frames  currents  of  warm 


FIG.  122. — Diagram  showing  enlarging  with  and  without  distortion:  A,  enlarging  without  dis- 
tortion; B,  distortion  for  rectification  of  print,  by  inclining  printing  surface;  C,  distortion,  for  recti- 
fication of  print,  by  inclining  both  negative  and  printing  surface. 

air  rise  from  pans  of  burning  alcohol,  previously  used  to 
soak  the  prints  and  now  useless  as  alcohol  because  of  their 
high  water  content. 

Before  putting  them  in  alcohol  it  is  advisable  to  squeegee 
all  the  surface  water  from  the  prints.    This  may  be  expedi- 


288        AIRPLANE  PHOTOGRAPHY 

tiously  done  by  removing  them  in  mass  from  the  final  wash 
water  upon  a  large  ferrotype  plate,  and  either  running  the 
plate  and  prints  together  through  a  wash  wringer  with  light 
pressure,  or  covering  the  whole  with  a  sheet  of  blotting  paper 
and  pressing  out  the  water  underneath  by  means  of  a  rubber 
squeegee  vigorously  applied. 

For  base  work  one  of  the  modern  automatic  print-drying 
machines  used  in  commercial  photography  would  be  desir- 
able. Glossy  surfaces  are  given  prints  by  the  usual  ferrotype 
plate  method.  But  this  is  too  time-consuming  for  war 
practice,  and  besides  has  but  doubtful  advantage  where 
papers  of  the  glossy  type  are  chosen. 


VI 

PRACTICAL  PROBLEMS  AND  DATA 


CHAPTER  XXV 
SPOTTING 

"  Spotting, ' '  as  distinct  from  mapping  or  from  the  photog- 
raphy of  continuous  strips,  is  the  photography  of  a  definite 
individual  objective.  In  military  work  spotting  or  "pin 
pointing"  includes  the  photography  of  particular  trenches 
or  pivotal  points  in  a  trench  system  before  an  attack  (Fig. 
123),  of  roads  or  bridges  along  which  an  advance  must  pass 
(Fig.  124),  of  batteries  or  big  guns  which  are  the  subject  of 
artillery  fire  (Fig.  125),  both  before  and  after  their  bombard- 
ment (Fig.  126),  of  gun  puffs  or  exploding  bombs  (Fig.  131). 

The  technique  of  spotting  consists  largely  in  getting  prop- 
erly over  the  target  and  then  securing  the  exposure  at  just 
the  right  moment.  This  is  chiefly  a  question  of  proper 
piloting;  but  the  aid  which  can  be  offered  to  the  pilot  by 
camera  auxiliaries  designed  particuarly  for  spotting  needs 
is  very  large. 

Discussion  of  the  task  of  the  pilot  who  must  steer  a 
photographic  plane  accurately  over  a  previously  selected 
point  of  interest  cannot  be  undertaken  without  raising  the 
question  of  who  should  take  the  picture,  pilot  or  observer? 
In  the  English  service  the  most  general  practice  was  for  the 
pilot  to  be  charged  with  the  responsibility  both  of  covering 
the  objective  and  of  exposing.  If  a  propeller  drive  was  used 
on  the  camera,  this  left  to  the  observer  only  the  task  of  chang- 
ing magazines.  If  the  camera  was  hand  operated  the  plates 
were  changed  either  by  the  observer,  or  else,  as  was  fre- 
quently the  case,  distance  operating  devices  were  attached, 
so  that  the  pilot  even  then  did  everything  except  change  the 
magazines,  and  the  observer  was  kept  free  to  watch  the  sky 

291 


292      AIRPLANE  PHOTOGRAPHY 


SPOTTING 


293 


AIRPLANE  PHOTOGRAPHY 


SPOTTING 


295 


for  enemy  aircraft.  A  very  desirable  adjunct  to  the  camera 
when  plates  are  shifted  automatically  or  by  the  observer  is 
a  distance  indicator,  to  show  the  pilot  when  the  shutter  is 
set.  Electrical  indicators  for  this  purpose  have  been  devised. 
If  the  camera  is  completely  hand  operated,  as  were  most 
of  those  in  the  French  and  German  services,  there  is  little 


FlQ.  126. — Example  of  spotting.     Battery  before  and  after  bombardment. 

choice  but  for  the  observer  to  perform  the  entire  operation. 
The  exposing  operation  could  have  been  delegated  to  the 
pilot,  but  such  was  not  the  custom  with  the  French  or  with 
the  American  squadrons  using  French  apparatus.  In  this 
method  of  operation  the  observer  depends  ,on  the  pilot  to 
get  the  plane  over  the  target,  while  the  pilot  depends  on  the 
observer  to  get  the  picture  when  the  target  is  covered. 
Ample  opportunity  is  thus  offered  for  misunderstanding 


296 


AIRPLANE  PHOTOGRAPHY 


and  disagreement.  This  can  be  avoided  only  by  excel- 
lent sights  properly  aligned,  for  both  pilot  and  observer, 
and  by  some  means  of  communication  between  the  two 
men  concerned. 

The  simplest  means  of  communication  is  of  course  direct 
conversation.    But  this  is  only  possible  in  those  planes,  such 


FlG.  127. — Photograph,  made  with  long  focus  lens  to  determine  the  results  of  aerial  bombing.    The 
"Tirpitz"  battery  of  long  range  naval  guns  directed  on  Dunkirk. 

as  the  DH  9,  in  which  pilot's  and  observer's  cockpits  are 
immediately  together,  so  that,  by  shouting,  any  desired 
information  can  be  conveyed  with  fair  ease.  When  the  dis- 
tance is  increased  to  four  or  five  feet,  as  in  the  DH  4,  the 
loudest  shouts  are  totally  lost  in  the  roar  of  the  engine  and 
the  blast  of  the  wind.  Speaking  tubes  and  telephones  are 
now  fairly  good,  but  are  none  too  comfortable  or  convenient 


SPOTTING 


297 


to  have  strapped  on  one's  head  and  face.  A  primitive 
device  used  to  some  extent  in  the  war  was  merely  a  pair  of 
reins  attached  to  the  pilot's  arms,  by  which  he  could  be 


f?/r  OBLIQUE  Ow/rt^  7v> 

V/0e  ffrf6LE  Of  L£rfS 
FIG.  128. — Diagram  showing  relationship  between  focal  length  and  area  covered  by  plate. 

directed  which  way  to  steer.  There  is  much  to  be  said  for  a 
simple  semaphore  system,  where  an  indicator  in  the  observer's 
cockpit  actuates  a  similar  dial  in  front  of  the  pilot,  indicating 
"right"  or  "left,"  "p:cture  obtained,"  "try  again,"  etc.  If 
the  observer  has  a  sight  by  which  he  can  see  far  enough 


298 


AIRPLANE  PHOTOGRAPHY 


ahead  to  correct  the  pilot's  error  of  pointing,  the  need  for  an 
accurate  sight  for  the  pilot  is  diminished. 

In  considering  the  question  of  sights,  attention  may 
again  be  called  to  the  poor  "visibility"  from  the  pilot's  seat 
in  the  present  prevailing  type  of  two-seater  tractor  plane. 


OVERLAP   PHOTOGRAPHS 


FIG.  129. — Diagram  giving  data  on  area  covered  at  various  altitudes  by  representative  lens. 

Blind  directly  in  front,  beneath,  and  to  either  side  (Figs.  7, 
8  and  9),  it  is  no  unusual  thing  for  a  pilot  to  entirely  miss  an 
objective,  such  as  a  railway  line,  which  he  can  only  estimate 
to  be  beneath  him  by  judging  its  distance  from  those  objects 
to  either  side  which  he  can  actually  see.  The  English  prac- 
tice of  leaving  a  clear  space  of  six  inches  to  a  foot  between  the 
fuselage  and  the  beginning  of  the  wing  fabric,  allows  the 
pilot  to  look  down  over  the  side,  a  decided  advantage.  But 


SPOTTING 


299 


for  photographic  purposes  nothing  can  compare  with  a  good 
negative  lens  carrying  fore  and  aft  lines  or  wires,  so  that  the 
pilot  can  see  his  object  ve  in  ample  time  to  head  directly  for 
it.  The  lens  should  either  be  large  enough  so  that  its  rear 


\.PlaceF.L  againsf /tight and 
read  the  Area  covered  against 
Arrow  A. 


cJ*laceArruwA  against  the  Area 

found  from  map  plotting  and  read 
rhe  HejghtagainstRcal Length. 


against  Hejghtand nole^i^^^^ 

Tjgure  against  Arrow  C.T^Iace  half  figure  againsf Arrow  C.PIoce  half  the 

the  around sbeed  aqainstmis  figure  ground  speed  doomsf /his  figure  and  the 


and  the  time  interval  for  overlaps^ 
is  seen  in  oufen 


time  interval  for  irertico/ stereos  is 
seen  in  outer  dial  opposite ArrowD 

Fia.  130. — Burchall  Slide  Bule,  for  calculating  intervals  between  exposures,  and  for  other  aerial 

photographic  data. 

edge  gives  the  view  directly  downward,  or  supplemented  by 
an  additional  lens  pointing  directly  down,  so  that  the  cover- 
ing of  the  target  is  assured.  To  locate  such  a  lens  in  the  front 
cockpit,  free  of  all  controls,  is  a  very  hard  task;  even  so  its 
view  is  likely  to  be  badly  interrupted  by  the  landing  gear. 
Nevertheless,  so  important  is  it,  both  in  photography  and  in 


300        AIRPLANE  PHOTOGRAPHY 

r    ..    "    lg£\'*f"'*       *'* 


FIQ.  ISl.-Aerial  bombardment  of  Trieste     Note  falling  bombs  in  center  of  picture;  and  exploding 
anti-aircraft  shells  over  the  water. 

Italian  official  photograph. 


SPOTTING 


301 


bombing,  to  have  a  sight  by  which  the  plane  can  be  accu- 
rately directed  that  designers  of  planes  should  recognize  this 
need  and  make  every  effort  to  provide  a  suitable  location. 

Sights  for  the  observer  have  been  discussed  already.  Here 
again  the  negative  lens  is  to  be  preferred,  but  while  the  pilot's 
lens  needs  only  directing  lines  in  the  axis  of  the  plane  (unless 


FIG.  132. — Example  of  spotting  requiring  exposure  at  exact  instant.     Explosion  following  burst  of 

bomb  in  ammunition  dump. 

British  official  photograph. 

he  takes  the  picture),  the  observer's  lens  needs  both  an  accu- 
rate center  mark  and  an  additional  upper  or  lower  sighting 
point.  Accurate  alignment  of  these  marks  with  the  camera 
axis  must  be  arranged  for  in  precise  spotting. 

Accurate  spotting  work  requiring  the  delineation  of  fine 
detail  calls  for  cameras  of  considerable  focal  length.  The 
camera  of  longest  focal  length  used  in  the  war  was  the  French 
120  centimeter  (Fig.  41).  This  was  employed  with  great 
success  in  such  work  as  regulating  the  fire  of  heavy  railway 


302 


AIRPLANE  PHOTOGRAPHY 


guns  brought  into  range  only  at  night,  to  fire  a  few  shots  at 
chosen  angles.  Photographs  taken  the  next  day  would  then 
show  the  exact  spot  where  each  shell  fell,  and  the  damage  it 
did,  to  serve  as  a  guide  for  the  next  night's  operations  (Fig. 
127).  The  field  of  these  cameras  is  quite  small — 8  to  12 
degrees — and  so  not  only  must  sighting  be  exact  but  the 


FIG.  133. — The  same  subject  a  few  minutes  later.    Height  of  smoke  shown  by  shadow. 
British  official  photograph. 

area  covered  on  the  ground  must  be  accurately  known.  This 
is  to  be  calculated  from  the  altitude,  focal  length,  and  plate 
size,  by  the  relation — 

distance  on  ground  _     altitude 
plate  length  focal  length 

Data  derived  from  such  calculations  may  be  incorporated 

in  tables,  or  graphically  in  diagrams  such  as  Figs.  128  and  129. 

These  calculations  and  others  required  in  mapping  and 

stereo-work   are   simply   and    quickly   made   by   slide-rule 


SPOTTING  SOS 

devices.  One  of  these,  the  Burchell  Photographic  Slide  Rule, 
developed  in  the  English  service,  is  shown  in  Fig.  130.  This 
consists  of  two  dials,  the  center  one  of  which  is  mounted — 
usually  by  a  pin  pushed  into  a  cork  behind — so  as  to  turn 
freely,  to  permit  its  being  set  for  altitude,  focal  length,  ground 
speed,  plate  size,  etc.,  whereupon  the  area  covered,  or  the 
appropriate  interval  between  exposures  may  be  read  off. 

Cameras  for  spotting  work  should  be  capable  of  exposure 
at  the  exact  moment  desired.  For  if  the  camera  is  ever  to 
catch  the  gun  as  it  discharges,  the  bomb  as  it  falls  (Fig.  131), 
or  the  shell  as  it  explodes  (Fig.  132),  the  photograph  must  be 
taken  within  the  instant.  Automatic  cameras,  exposing  at 
regular  intervals,  while  adequate  for  mapping,  are  not  fitted 
for  many  kinds  of  spotting. 


CHAPTER  XXVI 
MAP  MAKING 

Technique  of  Negative  Making. — Stated  in  its  simplest 
terms,  the  whole  problem  of  making  a  photographic  map 
from  the  air  consists  in  taking  a  large  number  of  slightly 
overlapping  negatives,  all  from  the  same  altitude,  with  the 
plane  flying  uniformly  level.  When  trimmed  and  mounted 
in  juxtaposition,  or  pasted  together  so  as  to  overlap  in  their 
common  portions,  the  prints  from  these  negatives  constitute 
a  complete  pictorial  map.  There  is  thus  furnished  by  a  few 
hours'  labor  topographic  information  which  would  be  the 
work  of  months  to  obtain  by  other  means. 

The  making  of  map  photographs  involves  all  the  special 
technique  of  spotting,  with  much  in  addition.  The  pilot's 
task  is  not  merely  to  go  over  one  object ;  he  must  navigate  a 
narrow  path,  at  a  constant  altitude,  on  an  even  keel.  If  he 
is  to  make  not  merely  a  ribbon,  but  a  map  of  considerable 
width,  he  must  take  successive  trips  parallel  to  the  first, 
each  displaced  just  far  enough  from  the  previous  course  to 
insure  that  no  portion  is  missed — a  difficult  task  indeed. 

It  is  the  observer's  duty  to  so  time  the  intervals  between 
exposures  that  they  overlap  enough,  but  not  so  much  as  to 
be  wasteful  of  plates  or  film.  He  must  also  change  maga- 
zines or  films  so  quickly  as  to  miss  no  territory,  or  if  some  be 
missed,  his  is  the  task  of  directing  the  pilot  back  to  the  point 
of  the  last  exposure,  where  they  begin  a  new  series. 

Level  flying  is  entirely  a  pilot's  problem.  Its  importance 
will  be  realized  when  we  consider  the  accompanying  diagrams 
(Figs.  134  and  135),  where  the  effect  on  the  resultant  picture 
is  shown  of  climbing,  gliding,  or  banking  to  either  side 

304 


MAP   MAKING 


305 


Prints  from  negatives  distorted  in  this  way  neither  will  be 
true  representations  of  the  territory  photographed,  nor  will 
they  match  when  juxtaposed.  In  fact,  they  can  be  utilized 
only  if  special  rectifying  apparatus  is  available  for  printing. 


LE.ISEL 


1LJ1 


FIG.  134. — Diagram  showing  effect  of  banking  on  aerial  photograph. 

Flying  at  a  constant  altitude  is  similarly  necessary  if  the 
prints  are  to  be  utilized  without  enlargement  or  reduction 
in  order  to  make  them  fit. 

Assuming  a  skilled  pilot  who  will  do  his  part,  the  next 
step  is  to  calculate  the  exposure  intervals  in  order  to  insure 
an  adequate  overlap.     If  a  negative  lens  is  installed  which 
20 


306 


AIRPLANE  PHOTOGRAPHY 


has  been  marked  with  a  rectangle  the  size  of  the  camera 
field,  the  simplest  method  is  to  estimate  the  proper  instant 
for  exposure  by  watching  the  progress  of  objects  across  the 
lens  face.  This  of  course  requires  constant  attention,  and 


FIG.  135. — Diagram  showing  effect  of  climbing  and  diving  on  aerial  photograph. 

it  is  easier  to  do  this  only  occasionally,  in  order  to  determine 
the  ground  speed  in  terms  of  camera  fields  traversed  per 
minute.  Thereafter  exposures  are  to  be  made  by  time,  as 
determined  by  a  watch  or  clock.  Any  desired  degree  of 
overlap  can  be  chosen,  and  either  estimated,  or  more  or  less 


MAP  MAKING  307 

accurately  fixed  by  lines  marked  on  the  negative  lens  at  a 
shorter  distance  apart  than  the  edges  of  the  field.  The 
most  usual  overlap  is  20  per  cent.,  except  for  stereos,  which 
call  for  50  to  75  per  cent. 

In  the  absence  of  a  negative  lens  or  some  other  sight  to 
show  the  whole  camera  field,  it  is  necessary  to  resort  to  cal- 
culation from  the  speed  and  altitude  of  the  plane,  the  focus 
of  the  lens  and  the  dimensions  of  the  plate.  If  A  is  the  alti- 
tude, a  the  focal  length  of  the  lens,  d  the  diameter  of  the 
plate  in  the  direction  of  travel  (usually  the  short  length  is 
chosen  for  economy  of  flights  to  cover  a  given  width),/  the 
fractional  part  by  which  one  negative  is  desired  to  overlap 
the  next,  and  V  the  ground  speed  of  the  plane,  then  we  have, 
by  simple  proportion,  that  the  interval  between  exposures, 
t,  must  be  — 


If  A  =  2000  meters,  d  =  lS  centimeters,  f=H,  a  =  50  centi- 
meters, and  V  =  200  kilometers  per  hour,  this  relation  gives  — 

2000  X  .18  X  .8  X  3600 


.5  X  200,000 


10.3  seconds 


The  principle  of  overlapping  map  exposures  is  shown  in  the 
accompanying  diagram  (Fig.  129),  together  with  data  cal- 
culated as  above  for  a  4X5  inch  plate. 

It  is  particularly  to  be  noted  that  it  is  the  ground  speed 
of  the  plane  that  is  used.  This  may  be  calculated  by  know- 
ing the  air  speed  and  the  wind  velocity  and  direction.  Fig. 
136  shows  the  method  of  doing  this  graphically.  First  an 
arrow  is  drawn  representing  the  direction  it  is  desired  to  fly. 
Next  a  second  arrow  is  drawn  of  length  to  represent  the  wind 
velocity.  This  must  be  inclined  toward  the  first  arrow  in 
the  direction  of  the  wind,  and  its  head  is  to  touch  the  head 
of  the  first  arrow.  Then  with  the  farther  end  of  this  second 


308        AIRPLANE  PHOTOGRAPHY 

arrow  as  a  center,  describe  a  circle  of  such  a  length  as  to 
represent  the  air  speed  of  the  plane,  in  the  same  units  as  the 
wind  velocity.  Connect  the  point  where  this  circle  cuts  the 
arrow  of  flight  direction  to  the  center  of  the  circle  by  a 
straight  line.  This  line  constitutes  the  air  speed  arrow, 
giving  the  direction  it  is  necessary  to  fly,  at  the  given  air 
speed,  to  make  the  course  desired.  The  length  of  the  flight 
direction  arrow  between  its  head  and  its  point  of  intersection 
with  the  air  speed  arrow  gives  the  ground  speed. 

When  the  wind  is  ahead  or  astern  this  calculation  reduces 
to  the  simple  subtraction  or  addition  of  the  wind  velocity 


Direction  vf  flight-                     ^ 
j<\       Ground  spetd    >| 

FIG.  136. — Diagram  showing  method  of  calculating  ground  speed  from  air  speed  and  wind  velocity. 

to  the  air  speed  of  the  plane.  Whenever  possible,  mapping 
should  be  done  up  and  down  the  wind  (Fig.  137).  If  the 
plane  is  "crabbing,"  the  above  calculations  for  overlap  are 
only  valid  if  the  camera  can  be  turned  normal  to  the  direc- 
tion of  travel  over  the  ground.  If  the  camera  cannot  be  so 
turned  the  corners  of  the  successive  pictures  overlap  instead 
of  their  sides,  with  quite  unsatisfactory  results  (Fig.  138). 

Calculation  of  the  distance  apart  of  the  parallel  flights 
necessary  to  make  a  map  of  any  width  is  done  by  the  use  of 
a  formula  similar  to  the  longitudinal  overlap  formula  above, 
distance  figuring  instead  of  time.  Using  the  same  symbols, 
and  denoting  the  distance  by  /),  we  have — 

D  =  W]-f) 

a 


MAP  MAKING 


309 


FIG.  137. — Overlaps  made  when  flying  with  or  against  the  wind. 


FIG.  138. — Unsatisfactory  overlaps  made  when  plane  is  "crabbing. 


310        AIRPLANE  PHOTOGRAPHY 

With  the  same  figures  as  before,  but  substituting  24  centi- 
meters for  the  plate  dimension,  this  relation  gives — 


_m 


It  is  of  course  largely  a  pilot's  problem  to  steer  the  plane 
over  parallel  courses  at  a  given  distance  apart,  although  the 
observer,  noting  conspicuous  objects  through  a  properly 
marked  negative  lens,  may  direct  the  pilot  by  any  of  the 
means  of  communication  already  mentioned. 

An  alternative  method  of  securing  parallel  strips,  which 
is  to  be  highly  recommended  where  enough  photographically 
equipped  airplanes  are  available,  is  for  several  planes  to  fly 
side  by  side,  maintaining  their  proper  separation  (Fig.  139). 

Cameras  and  Auxiliaries  for  Map  Making. — Mapping  can 
be  done  quite  satisfactorily  by  hand  operated  or  semi- 
automatic cameras,  provided  the  observer  has  not  too  many 
other  duties.  On  the  other  hand,  the  operation  of  exposing 
at  more  or  less  definite  intervals  of  time,  irrespective  of  the 
object  immediately  presented  to  the  camera,  is  a  largely 
mechanical  one.  It  naturally  suggests  the  employment  of  an 
automatic  mechanism,  whose  speed  of  operation  only  is  it 
necessary  to  watch. 

If  a  non-automatic  camera  is  used  the  timing  of  exposures 
may  be  done  by  watching  a  negative  lens,  as  described  above, 
or  by  reference  to  a  clock,  assuming  that  the  ground  speed  is 
known  through  calculation.  A  very  practical  advance  over 
the  ordinary  use  of  a  clock  is  to  attach  a  stop-watch  to  the 
shutter  release,  so  that  it  is  turned  back  to  zero  and  re-started 
at  each  exposure  (Fig.  70) .  In  passing,  it  may  be  noted  that 
if  the  stop-watch  hand  makes  an  electric  contact  which  throws 
the  shutter  release,  then  the  device  constitutes  an  attachment 
for  turning  any  semi-automatic  camera  into  an  automatic. 


MAP  MAKING  311 

The  most  suitable  cameras  for  mapping  are  unquestion- 
ably those  of  the  entirely  automatic  type.  The  use  of  such 
cameras  always  demands  a  knowledge  of  the  ground  speed. 


FIG.  139. — Planes  starting  out  to  make  a  map  by  flying  in  parallel. 

This  demand  has  led  to  many  suggestions  for  ground  speed 
indicators.  The  common  idea  of  these  is  to  provide  a  moving 
part  on  the  plane — either  a  disc  of  large  diameter,  or  a  chain, 
or  a  revolving  screw — whose  speed  may  be  varied  until  any 
point  upon  it  appears  to  keep  in  coincidence  with  a  point  on 


312        AIRPLANE  PHOTOGRAPHY 

the  moving  landscape  below.  The  ground  speed  is  then  to 
be  read  off  a  properly  calibrated  dial.  Or,  as  a  further  step, 
the  frequency  of  the  exposures  may  be  directly  controlled 
by  the  ground  speed  indicator  mechanism.  The  entire 
control  of  the  camera  would  then  consist  merely  in  occasional 
adjustment  of  the  ground  speed  indicator. 

While  entirely  possible  in  theory,  these  devices  are  not 
easy  to  work  with  in  practice,  because  the  plane  is  always 
subject  to  some  pitching  and  rolling,  which  make  it  difficult 
to  hold  any  object  constantly  on  the  moving  point.  This  is 
especially  true  at  high  altitudes,  where  the  apparent  motion 
of  the  earth  is  quite  slow  compared  to  the  swervings  of  the 
plane.  This  objection  is  in  part  removed  if  the  ground  speed 
indicator  is  carried  by  a  gyro  stabilizer. 

Ordinary  mapping  does  not  demand  such  exquisite 
rendering  of  detail  as  does  trench  mapping.  Nor  is  it  neces- 
sary to  fly  in  peace-time  at  such  high  altitudes  as  in  war. 
In  consequence,  mapping  cameras  are  preferably  of  the  short 
focus,  wide  angle  type,  say,  25  centimeter  focus  for  an  18  X  24 
centimeter  plate.  Film  is  to  be  preferred  over  plates  because 
of  the  greater  number  of  exposures  it  is  possible  to  make  on  a 
flight.  The  shutter  of  the  mapping  camera  must  be  ex- 
tremely uniform  in  its  rate  of  travel  so  that  the  elements  of 
the  map  may  match  in  tone  (Fig.  140).  A  mount  which 
permits  the  camera  to  be  turned  normal  to  the  direction  of 
flight,  such  as  the  British  turret  mount  (Fig.  87),  is  particu- 
larly desirable  if  flying  across  the  wind  is  necessary,  as  will 
often  be  the  case  in  mapping  strips  between  towns  or  between 
flying  fields.  Devices  to  indicate  compass  direction  and 
altitude  are  called  for  in  new  and  poorly  mapped  territory, 
and  may  be  expected  to  receive  intensive  study  in  the  future. 
The  question  of  their  utility  is,  however,  bound  up  with  the 
whole  question  of  the  sphere  of  aerial  photographic  mapping. 


MAP  MAKING 


313 


314        AIRPLANE  PHOTOGRAPHY 

Up  to  the  present  this  has  been  almost  entirely  a  matter  of 
filling  in  details  on  maps  obtained  by  the  regular  surveying 
methods,  or  of  making  pictorial  maps  for  aviators.  To  what 
extent  primary  mapping  can  be  done  by  the  airplane  is  yet 
to  be  determined. 

At  this  point  mention  must  be  made  of  special  cameras 
for  securing  extremely  wide  angle  views,  thereby  minimizing 
the  number  of  flights.  The  Bagley  camera,  devised  by  Major 
Bagley  of  the  U.  S.  Engineers,  is  an  example.  It  has  three 
lenses,  a  middle  one  pointing  directly  downward,  and  one  to 
either  side  at  an  angle  of  35  degrees.  The  pictures  obtained 
with  the  side  cameras  are  of  course  greatly  distorted,  and 
must  be  rectified  in  a  special  rectifying  camera.  The  result- 
ant definition  is  not  good,  but  as  the  maps  are  made  on  a 
much  smaller  scale  than  the  original  pictures,  this  is  not  a 
serious  objection.  It  is  a  matter  for  the  future  to  decide 
whether  the  additional  labor  on  the  ground  necessary  for 
the  rectifying  process  is  to  be  more  expensive  than  the  extra 
flights  which  must  be  made  with  the  ordinary  types  of  cam- 
eras covering  a  smaller  angle. 

Printing  and  Mounting  Mosaics. — With  an  ordinary  set 
of  overlapping  negatives  the  first  step  toward  producing  a 
map  is  to  scale  the  negatives.  For  this  purpose  one  should 
be  selected  which  by  comparison  with  a  map  shows  no  dis- 
tortion, and  which  is  on  the  desired  scale,  or  is  known  to 
have  been  made  at  the  average  altitude  of  flight.  A  sketch 
map  of  the  territory  should  then  be  drawn,  on  this  scale, 
based  on  available  maps.  This  sketch  is  preferably  made 
on  a  large  ground  glass  illuminated  from  behind  (Fig.  141). 
On  this  all  the  negatives  should  be  laid,  and  their  proper 
relative  positions  sought.  When  this  is  done  it  is  evident  at 
once  whether  all  the  territory  has  been  covered,  and  whether 
there  are  any  superfluous  negatives.  Each  negative  should 


MAP  MAKING 


315 


then  be  examined  as  to  its  scale  and  distortion.  If  it  can  be 
made  to  fit  the  scale  by  simple  enlargement  or  reduction,  a 
line  can  be  drawn  on  one  edge  of  a  length  indicating  its 
scale.  This  line  will  later  be  used  as  a  guide  in  the  enlarging 
camera.  If  the  picture  is  badly  distorted  it  must  either  be 
replaced  by  another  negative,  or  if  rectifying  apparatus 
is  available,  it  must  be  set  aside  for  the  making  of  a 
rectified  print. 


FIG.  141. — Scaling  negatives  for  mosaic  map-making. 

The  next  step  is  to  make  prints  from  the  negatives,  which 
may  be  done  either  by  contact,  or,  necessarily  if  differences 
of  scale  must  be  compensated,  in  the  enlarging  camera.  If 
prints  to  an  exact  scale  are  required  the  shrinkage  of  the 
paper  must  be  determined  and  allowed  for.  The  prints  must 
all  show  the  same  tone,  and  must  be  uniform  from  edge  to 
edge.  If  the  focal-plane  shutter  is  not  uniform  in  its  travel, 
as  is  frequently  the  case,  this  means  that  the  print  must  be 
"dodged,"  or  exposed  more  at  one  edge  than  the  other,  by 
locally  shielding  the  plate  and  paper  during  exposure.  A 


316        AIRPLANE  PHOTOGRAPHY 

case  of  the  step-like  effect  caused  by  uneven  shutter  action 
is  shown  in  Fig.  140.  The  effect  due  to  uneven  shutter  action 
is  of  course  absent  with  a  between-the-lens  shutter,  which 
constitutes  a  strong  argument  in  favor  of  that  type  for  use 
in  mapping  cameras. 

When  the  prints  are  made  they  must  be  mounted  together 
on  a  large  card  or  cloth  background.     For  a  very  small 


FIG.  142. — Arranging  prints  for  a  mosaic  map. 

mosaic  they  may  'be  juxtaposed  by  simple  examination, 
matching  corresponding  details  in  successive  prints.  For  a 
mosaic  of  any  size  an  accurate  outline  map  must  be  drawn 
on  the  surface  to  which  the  prints  are  to  be  attached.  The 
prints  are  then  laid  out  on  this  outline,  moved  to  their 
correct  positions,  and  held  down  by  pins  (Fig.  142).  When 
they  are  all  arranged  the  final  mounting  may  be  begun. 
The  excess  paper,  beyond  what  is  necessary  for  safe  overlaps, 


MAP  MAKING  317 

may  be  trimmed  off,  exercising  judgment  as  to  which  print 
of  each  adjacent  pair  is  of  the  better  quality,  and  utilizing 
it  for  the  top  one  at  the  overlapping  junction.  If  one  print 
shows  serious  distortion  it  may  be  placed  under  its  fellows 
on  all  four  edges,  thus  minimizing  its  weight.  The  edges 
are  best  made  irregular  by  tearing.  Straight  edges  are  apt 
to  force  themselves  on  one's  attention  in  the  final  mosaic 
and  give  an  erroneous  impression  of  the  existence  of  straight 
roads  or  other  features.  Both  forms  of  edging  are  shown  in 
Figs.  124  and  143. 

An  alternative  method  of  securing  the  final  print  mosaic, 
where  film  negatives  are  used,  is  to  trim  successive  film 
negatives  so  that  the  trimmed  sections  will  exactly  juxtapose, 
instead  of  overlap.  The  sections  are  then  mounted,  by 
stickers  at  their  edges,  on  a  large  sheet  of  glass,  and  printed 
together.  Captured  German  prints  show  that  this  was  the 
method  commonly  used  with  the  German  film  camera  (Fig.6&) . 

It  will  be  noted  that  the  procedure  which  has  been 
described  and  illustrated  by  Figs.  142  and  143  assumes  the 
previous  existence  of  a  map  accurately  placing  at  least  the 
chief  features  of  the  country  covered.  This  draws  attention 
at  once  to  the  limitations  and  true  sphere  of  aerial  photo- 
graphic mapping  at  the  present  time.  With  the  cameras 
thus  far  it  is  not  possible,  nor  is  it  attempted,  to  do  primary 
mapping  of  unknown  regions.  Distortions  due  to  lens, 
shutter,  film  warping  and  paper  shrinkage  considerably 
exceed  the  figures  permitted  in  precision  mapping.  From 
the  standpoint  of  geodetic  accuracy  the  cumulative  errors 
of  deviations  in  direction,  altitude  and  level,  peculiar  to 
flying,  would  soon  become  prohibitive. 

The  great  field  for  aerial  photographic  mapping  in  the 
near  future  lies  in  filling  in  detail  on  maps  heretofore  com- 
pleted as  to  general  outlines,  cr,  as  in  the  war,  en  maps  far 


318        AIRPLANE  PHOTOGRAPHY 


MAP  MAKING  319 

out  of  date.  The  war-time  procedure  in  country  largely 
unknown,  such  as  Mesopotamia,  was  probably  closely  that 
which  will  be  necessary  in  peace.  Conspicuous  points  in  the 
landscape  were  first  triangulated  from  friendly  territory,  and 
from  these  the  outline  map  was  drawn,  whose  details  were 
to  be  supplied  by  aerial  photographs.  Much  of  the  "map- 
ping" of  cross  country  aerial  routes  so  far  done  is  frankly  of 
a  pictorial  nature,  showing  conspicuous  landmarks  and  good 
landing  fields — extremely  valuable  and  useful,  but  not  to  be 
confused  with  precision  mapping.  In  assembling  mosaics  of 
this  kind  the  elaborate  procedure  described  above  is  not 
followed.  The  process  is  the  simple  one  of  juxtaposing 
adjacent  prints  as  accurately  as  possible  by  visual  examina- 
tion. Errors  are  of  course  cumulative,  but  as  long  as  exact 
distances  are  not  in  question  this  is  no  matter. 


CHAPTER  XXVII 
OBLIQUE  AERIAL  PHOTOGRAPHY 

Oblique  views  from  the  airplane  are  of  very  great  value. 
While  vertical  views  are  more  searching  in  many  respects, 
they  do  nevertheless  present  an  aspect  of  the  earth  with 
which  ordinary  human  experience  is  unfamiliar.  Conse- 
quently they  are  difficult  to  interpret  without  special  train- 
ing. They  suffer,  too,  from  the  military  standpoint,  from  the 
limitation  that  it  is  with  vertical  extension  just  as  much  as 
with  horizontal  that  an  army  has  to  contend  in  its  progress. 
Elevations  and  depressions  of  land  show  on  an  oblique  view 
where  they  would  be  entirely  missed  in  a  vertical  one.  For 
illustration,  study  the  picture  of  part  of  the  outskirts  of 
Arras  (Fig.  144),  presenting  moat,  walls  and  embankments, 
all  of  which  would  be  serious  obstacles,  but  would  hardly  be 
noticed  on  a  vertical  view.  Pictures  taken  from  directly 
overhead  are  eminently  suited  to  artillery  use,  but  oblique 
views  of  the  territory  to  be  attacked,  taken  from  low  alti- 
tudes, formed  an  essential  part  of  the  equipment  of  the 
infantry  in  the  later  stages  of  the  war. 

Pictorially,  oblique  views  are  undoubtedly  the  most 
satisfactory.  The  most  revealing  aspect  of  any  object  is 
not  one  side  or  face  alone,  but  the  view  taken  at  an  angle, 
showing  portions  of  two  or  three  sides.  Best  of  all  is  that 
taken  to  show  portions  of  front,  side  and  top — the  well- 
known  but  heretofore  fictitious  "bird's-eye  view"  (Fig.  145). 
This  possibility  is  ordinarily  denied  the  surface-of-the-earth 
photographer,  but  the  proper  vantage  point  is  attained  in 
the  airplane. 

Aerial  obliques  may  be  taken  at  any  angle,  although  a 


OBLIQUE  PHOTOGRAPHY 


321 


distinction  is  sometimes  made  between  obliques  of  high 
angle  and  panoramic  or  low  angle  views  (Fig.  146).  In 
addition  to  ordinary  obliques,  a  very  beautiful  development 
is  the  stereo  oblique.  Both  kinds  of  oblique  photography 
call  for  special  instrumental  equipment  and  technique. 


FIG.  144.— The  outskirts  of  Arras.    Low  oblique  showing  contours. 

Methods  and  Apparatus  for  Oblique  Photography. — The 

simplest  method  of  taking  oblique  pictures  from  a  plane  is 
to  use  a  hand  camera  pointed  at  the  desired  angle.  Its  limi- 
tations are  in  the  size  and  scale  of  the  picture  obtainable, 
and  in  the  inherent  limitations  to  the  method  of  camera 
support.  A  step  in  advance  of  this  is  to  mount  the  camera 
above  the  fuselage,  on  the  machine  gun  ring  or  turret,  in 
place  of  the  gun.  Considerably  greater  rigidity  is  thus 

21 


AIRPLANE  PHOTOGRAPHY 


OBLIQUE  PHOTOGRAPHY 


323 


324        AIRPLANE  PHOTOGRAPHY 

obtained,  and  heavier  cameras  can  be  utilized,  although  the 
wind  resistance  is  a  serious  factor.  Excellent  obliques  have 
been  made  in  this  way,  even  with  50-centimeter  cameras, 
but  the  scheme  is  impractical  in  military  planes,  because  of 
the  removal  of  machine  gun  protection. 

If  the  camera  is  fixed  in  the  fuselage  in  its  normal  vertical 
position,  obliques  may  be  and  have  been  taken  by  the  simple 
expedient  of  banking  the  plane  steeply.  This  is  not  to  be 
recommended  as  a  standard  procedure,  especially  for  taking 
a  consecutive  series  of  exposures. 

The  most  satisfactory  arrangements  for  taking  obliques 
are  two;  first,  to  mount  the  camera  obliquely  in  the  plane,  and 
second,  to  use  a  mirror  or  prism,  in  front  or  behind  the  lens 
of  the  vertically  mounted  camera.  The  first  method 
has  been  employed  chiefly  by  the  French,  the  latter 
by  the  English,  whose  gravity  fed  cameras  could  not  be 
mounted  obliquely 

Taking  up  first  the  oblique  mounting  of  cameras,  we 
find  two  ways  of  doing  this:  longitudinal  mounting  and 
lateral  mounting.  In  longitudinal  mounting  the  camera 
projects  forward  and  downward,  usually  from  the  nose  of  a 
pusher  or  bi-motored  plane.  With  this  form  of  mounting 
(Fig.  147)  it  is  necessary  of  course  to  fly  directly  toward  the 
objective.  If  this  is  a  portion  of  enemy  trench,  which  must 
be  photographed  from  a  height  of  400  or  500  meters,  the 
plane  will  be  directly  on  top  of  its  objective  a  few  seconds 
after  the  exposure  is  made,  and  be  a  conspicuous  target,  in 
imminent  danger  of  destruction.  Moreover,  only  a  single 
short  section  of  the  trench  would  be  obtained  for  each  cross- 
ing of  the  line.  The  one  case  where  resort  to  this  method  is 
practically  forced  is  with  the  120-centimeter  cameras  which 
simply  cannot  be  slung  athwart  the  plane.  There  is  a  slight 
advantage  in  this  method  of  carrying  in  that  the  motion  of 


OBLIQUE  PHOTOGRAPHY 


325 


the  image  is  less  if  the  objective  is  approached,  instead  of 
being  passed  at  the  side,  and  so  longer  exposures  can  be  made. 
The  longitudinal  mounting  has,  however,  been  very  generally 
superseded  by  the  lateral. 

Methods  for  mounting  cameras  obliquely  for  taking  pic- 
tures through  the  side  of  the  plane  have  been  discussed  in 
detail  in  connection  with  camera  mountings  and  installations 
(Fig.  93).  The  chief  difficulties  are  want  of  space,  obstacles 


FIG.  147. — 120-centimeter  camera  mounted  obliquely  in  the  fore-and-aft  position. 

at  the  side  such  as  control  wires  and  longerons,  and  failure 
of  the  camera  to  function  properly  at  an  angle.  Even  in  the 
broad  circular  sectioned  fuselage  of  the  Salmson  plane, 
quarters  are  so  cramped  that  the  French  50-centimeter 
camera  when  obliquely  mounted  cannot  be  used  with  the 
12-plate  magazine,  and  recourse  is  made  to  thin  flat  double 
plate-holders.  Holes  in  the  side  of  the  fuselage  should. clear 
all  wires  and  should  command  a  view  unobstructed  by  the 
wings — which  often  means  that  the  camera  must  be  carried 
behind  the  observer's  cockpit,  irrespective  of  the  suitability 


AIRPLANEPHOTOGRAPHY 


of  that  space  from  other  standpoints.  Cameras  dependent 
for  their  action  on  gravity,  such  as  the  deRam  and  English 
L  type,  are  unsuited  for  oblique  suspension. 

For  cameras  which,  because  of  their  method  of  operation 

or  shape  cannot  be  slung  ob- 
liquely, the  only  way  to  take 
obliques  is  to  employ  mirrors 
(Fig.  148)  or  prisms.  These 
must  be  of  the  same  optical 
quality  as  the  photographic 
lens.  They  are  both  necessarily 
of  considerable  weight  because 
they  must  be  of  large  area  of 
face  to  fill  the  entire  aperture 
of  an  aerial  lens.  Mirrors  are 
lighter  than  prisms,  but  must 
be  quite  thick  to  prevent  dis- 
tortion of  the  surface  due  to  any 
possible  strains  to  their  mount. 
Right  angle  glass  prisms  have 
been  used  by  the  English  with 
the  8  and  10  inch  L  cameras. 
^H  The  prisms  were  uniformly 

Ik:       \  tilted  to  an  angle  of  12^  de- 

^          grees  from  the  horizontal. 
^  Glass  mirrors  can  be  silvered 

either  on  the  rear  or  front  sur- 
face. If  on  the  rear,  both 
surfaces  must  be  accurately 
parallel,  which  means  much  greater  labor  and  expense  than 
if  the  front  surface  can  be  utilized.  The  difficulty  with  front 
surface  mirrors  is  that  the  metallic  coating  is  easily  tarnished 
or  scratched,  especially  if  silver  is  used,  which  is  almost 


FIG.  148. — Mirror  on  camera  cone  for  taking 
oblique  views. 


OBLIQUE  PHOTOGRAPHY          327 

imperative,  since  all  the  other  metals  have  considerably 
lower  reflecting  powers.  (Gold  might  serve  both  as  mirror 
and  color  filter,  because  of  its  yellow  color.)  Placing  the 
mirror  inside  the  camera  body  in  part  obviates  this  trouble, 
but  means  the  use  of  a  special  elbow  lens  cone.  In  any  case 
the  mirror  or  prism  occasions  at  least  a  10  per  cent,  loss  of 
light.  Pictures  taken  by  reflectors  of  any  kind  are  reversed, 
and  must  either  be  printed  in  a  camera,  or  on  transparent 
film  which  may  be  viewed  from  the  back. 

The  most  usual  condition  for  making  obliques  is  to  fly 
very  low  (300  to  600  meters),  with  the  line  of  sight  of  the 
camera  from  12  to  45  degrees  from  the  horizontal.  This 
low  altitude  necessitates  very  short  exposures,  to  avoid  move- 
ment of  the  image.  The  picture  may  be  taken  either  the  long 
or  the  short  way  of  the  plate,  depending  on  the  character  of 
the  object  and  the  information  desired.  It  is  to  be  noted 
that  successive  oblique  pictures  cannot  be  mounted  to  form 
a  continuous  panorama — this  being  possible  with  obliques 
only  if  they  are  taken  from  one  point,  as  from  a  captive 
balloon.  If  successive  views  are  made  on  a  straight  flight 
at  intervals  so  as  to  exactly  juxtapose  in  the  foreground, 
they  overlap  by  a  large  margin  the  middle,  and  a  point  on 
the  horizon,  if  that  shows,  will  be  in  the  same  position  in 
every  picture.  Mosaics  of  obliques  could  be  made  only  by 
some  system  of  conical  mounting. 

Sights  for  Oblique  Photography. — Any  of  the  sights 
previously  discussed  for  vertical  work,  such  as  the  tube 
sights,  are  applicable  to  obliques.  They  must,  however,  be 
suited  for  mounting  at  an  angle,  in  a  position  convenient  for 
the  observer.  In  addition,  provision  must  be  made  for 
adjusting  the  angle  so  that  the  lines  of  sight  of  camera  and 
finder  are  parallel.  Mounting  outside  the  fuselage  is  prac- 
tically the  only  feasible  way,  and  is  less  objectionable  with 


328         AIRPLANE  PHOTOGRAPHY 

oblique  than  with  vertical  sights,  as  oblique  sighting  does 
not  require  the  observer  to  stand  up  and  lean  over  the  edge 
of  the  cockpit.  Windows  in  the  side  of  the  fuselage,  either 
of  celluloid  or  non-breakable  glass,  are  a  great  aid  to  oblique 
observation.  Marks  upon  the  transparent  surface  can  be 
utilized  for  the  rear  points  of  a  sight  of  which  the  front  point 
is  a  single  fixed  bead  or  rectangle. 


CHAPTER  XXVIII 
STEREOSCOPIC  AERIAL  PHOTOGRAPHY 

One  of  the  most  striking  and  valuable  developments  in 
aerial  photography  has  been  the  use  of  stereoscopic  views. 
Pairs  of  pictures,  taken  with  a  considerable  separation  in 
their  points  of  view  and  studied  later  by  the  aid  of  the  stereo- 
scope, show  an  elevation  and  a  solidity  which  are  entirely 
wanting  in  the  ordinary  flat  aerial  vista.  Often,  indeed,  these 
attributes  are  essential  for  detecting  and  recognizing  the 
nature  of  objects  seen  from  above.  Stereoscopic  aerial 
photography  has  been  justly  termed  "the  worst  foe  of 
camouflage." 

Principles  of  Stereoscopic  Vision. — The  ability  to  see 
objects  in  relief  is  confined  solely  to  man  and  to  a  few  of  the 
higher  animals  in  whom  the  eyes  are  placed  side  by  side. 
When  the  eyes  are  so  placed  they  both  see,  to  a  large  extent, 
the  same  objects  in  their  fields  of  view.  Owing  to  the  sepa- 
ration of  the  eyes  the  actual  appearance  of  all  objects  not  too 
far  away  is  different,  and  it  is  by  the  interpretation  of  these 
differences  that  the  brain  gets  the  sensation  of  relief.  Thus 
in  Fig.  149  the  two  eyes  are  shown  diagrammatically  as 
looking  at  a  cube.  The  right  eye  sees  around  on  the  right- 
hand  face  of  the  cube,  the  left  eye  on  the  left-hand  face  of 
the  cube.  The  two  aspects  which  are  fused  and  interpreted 
by  the  brain  are  shown  in  the  lower  diagram. 

Stereoscopic  views  or  stereograms,  made  either  by  pho- 
tography or,  in  the  early  days,  by  careful  drawing,  consist  of 
pairs  of  pictures  made  of  the  same  object  from  two  different 
points.  For  ordinary  stereoscopic  work  these  points  are  sepa- 
rated by  the  distance  between  the  eyes,  approximately  65  mil- 

329 


330 


AIRPLANE  PHOTOGRAPHY 


limeters  or  2^  inches.    These  two  pictures  are  then  so  viewed 

that  each  eye  receives  its  appropriate  image  from  the  proper 

direction,  whereupon  the  object  delineated  stands  out  in  relief. 

Fusion  of  the  two  elements  of  the  stereoscopic  picture 

can  take  place  without  the  assist- 
ance of  any  instrument,  if  the 
eyes  are  properly  directed  and 
focussed,  but  this  comes  only  with 
practice.  Holding  the  stereogram 
well  away  from  the  face  the  eyes 
are  directed  to  a  distant  object 
above  and  beyond,  in  order  to 
diverge  the  axes.  Then  without 
converging,  the  eyes  are  dropped 
to  the  picture,  which  should  spring 
into  relief.  It  is  necessary  in 
moving  the  eyes  from  the  distant 
object  to  the  near  stereogram  to 
alter  their  focus  somewhat,  de- 
pending on  how  near  the  stereo- 
gram  is  held;  and  the  success  of 
the  attempt  to  fuse  the  images 
depends  on  the  observer's  ability 
to  maintain  the  eyes  diverged  for 
a  distant  object  while  focussing  for 
a  near  one.  Near-sighted  people 
(ontakingoff  their  glasses)  fuse  the 
FIG.  HO— The  principle  of  stereoscopic  stereoscopic  images  quite  easily, 

vision.  .  ° 

since  their  eyes  do  not  locus  on 

distant  objects  even  when  diverged  for  them.  Transparen- 
cies are  easier  to  fuse  than  paper  prints,  but  in  any  case  where 
a  stereoscope  is  not  used  the  separation  of  image  centers 
should  not  be  more  than  that  of  the  eyes. 


STEREOSCOPIC  PH  OTO  GRAPH  Y331 

Stereoscopes. — The  easier  and  more  usual  method  of 
fusing  the  stereoscopic  images  is  by  a  stereoscope.  The 
simplest  form  consists  merely  of  two  convex  lenses,  one  for 
each  eye,  their  centers  separated  by  a  distance  somewhat 
greater  than  that  between  the  eyes.  Their  function  is  to 
bring  the  stereogram  to  focus,  and,  by  the  prismatic  action 
of  the  edges  of  the  lenses,  to  converge  the  lines  of  sight 

-    '3 


FIG.  150 


form  of  prism  stereoscope. 


which  pass  through  the  centers  of  the  two  pictures  to  a  point 
in  space  in  front  of  the  observer.  The  two  lenses  should  be 
mounted  so  as  to  provide  for  the  adjustment  of  their  sepa- 
ration to  fit  different  eyes  and  print  spacings.  The  most  com- 
mon form  of  stereoscope  is  that  designed  by  Holmes,  for 
viewing  paper  print  stereograms  (Fig.  150).  It  has  pris- 
matic lenses  of  an  appropriate  angle  to  converge  pictures 
whose  centers  are  three  inches  apart,  instead  of  the  lesser 
distance  appropriate  to  stereograms  intended  for  fusing 


332 


AIRPLANE  PHOTOGRAPHY 


without  an  instrument.  No  adjustment  is  provided  for 
varying  the  lens  separation,  but  the  print  can  be  moved  to 
and  fro  for  focussing. 

Another  form  of  stereoscope,  one  of  the  first  produced,  is 
the  mirror  stereoscope  (Fig.  152),  now  used  extensively  for 


FIG.  151. — Box  stereoscope. 

viewing  stereo  X-ray  pictures.  It  consists  of  two  vertical 
mirrors  at  right  angles  to  each  other,  with  their  edge  of 
contact  between  the  eyes.  The  two  prints  to  be  studied  are 
placed  to  right  and  left,  an  arrangement  that  permits  the 
use  of  prints  of  any  size.  The  convergence  point  is  controlled 
by  the  angle  between  the  mirrors.  The  Pellin  stereoscope 


STEREOSCOPIC   PHOTOGRAPHY 333 

(Eig.  153)  utilizes  two  pairs  of  mirrors  in  a  way  to  permit  the 
use  of  large  prints.  The  prints  are,  however,  placed  side  by 
side  on  a  horizontal  viewing  table,  which  avoids  certain 
difficulties  of  illumination  met  with  in  the  simpler  mirror 
form.  The  box  form  of  stereoscope  (Fig.  151)  using  either 
prisms  or  simple  convex  lenses,  is  particularly  adapted  for 
viewing  transparencies,  although  the  insertion  of  a  door  at 
the  top  provides  illumination  for  paper  prints.  The  Schweiss- 


FIG.  152. — Diagram  of  mirror  stereoscope. 

guth  design  (Fig.  154)  is  intended  primarily  as  an  aid  to 
selecting  the  portions  of  the  prints  to  be  cut  out  for  mounting. 
The  platform  on  which  the  pictures  rest  is  composed  of  two 
long  rectangular  blocks,  on  which  are  plates  of  glass  raised 
sufficiently  to  permit  the  prints  to  be  slid  underneath.  The 
space  between  the  blocks  allows  the  unused  portion  of  the 
photograph  to  be  turned  down  out  of  the  way.  Prints  of 
any  size  can  thus  be  moved  about  until  the  proper  portions 
for  stereo  mounting  are  found.  Either  block  can  be  moved 


334        AIRPLANE  PHOTOGRAPHY 

in  its  own  plane  and  also  to  and  from  the  eye,  whereby  two 
prints  of  somewhat  different  scales  can  be  fused. 

The  Taking  of  Aerial  Stereograms. — The  normal  separa- 
tion of  the  eyes  is  altogether  too  small  to  give  an  appearance 
of  relief  to  objects  as  far  away  as  is  the  ground  from  a  plane 
at  ordinary  flying  heights.  In  order  to  secure  stereoscopic 
pairs  it  is  therefore  necessary  to  resort  to  a  method  originally 


FIG.  153. — Pellin  double  mirror  stereoscope. 

employed  for  photographing  distant  mountains  and  clouds. 
This  is  to  take  the  two  pictures  from  points  separated  by 
distances  much  greater  than  the  inter-ocular  separation — by 
meters  instead  of  millimeters — corresponding  to  the  positions 
of  the  eyes  on  a  veritable  giant.  In  the  airplane  this  is  accom- 
plished by  making  successive  exposures  as  the  plane  flies  over 
the  objective,  at  intervals  to  be  determined  by  the  speed,  the 
altitude  and  the  amount  of  relief  desired  (Fig.  155). 

An  all  important  question  which  arises  immediately  is: 
What  separation  of  points  of  view  shall  we  select?    If  the 


STEREOSCOPIC  PHOTOGRAPHY 335 

exposures  are  too  close  together  there  will  be  little  relief; 
if  too  distant  the  relief  will  be  so  great  as  to  be  unnatural, 
even  offensive.  Obviously  we  cannot  here  establish  a  cri- 
terion of  natural  appearance,  since  the  natural  appearance 


FIG.  154. — Schweissguth  stereoscope,  used  for  selecting  portions  of  prints  to  be  mounted. 

to  ordinary  human  eyes  is  devoid  of  relief.  We  may,  how- 
ever, define  correct  relief  as  that  obtained  when  the  apparent 
height  of  elevated  objects  is  right  as  compared  with  their  extension 
or  plan. 

In  order  to  secure  this  condition  it  is  necessary,  first,  that 
each  element  of  the  stereoscopic  pair  be  correct  in  its  per- 


336 


AIRPLANE    PHOTOGRAPHY 


spective.  This  is  fortunately  an  old  photographic  problem, 
already  well  understood.  Its  solution  is  to  view  the  photo- 
graph from  a  distance  exactly  equal  to  the  focal  length  of  the 
camera  lens.  Since  the  normal  viewing  distance  is  not  less 
than  25  centimeters,  lenses  of  this  focal  length  at  least  are 
requisite  for  correct  perspective.  Secondly,  it  is  necessary 
for  correct  relief  that  the  two  views  be  taken  with  a  separa- 
tion equal,  on  the  plane  of  the  plate,  to  the  separation  of  the 


STEREOSCOPIC  PHOTOGRAPHS 


TffBLES 


WITH 


flegg  Core/gen-  He/ttf? 
2063  xf  666  -fOflOO 
I675*t500~  9.000 
/666X/333  -   $000 
/45&K//66-   7.000 
IZSOtfOOO-   6,000 
tO4f  X  633  •    5.OOO 
633X  666  •  4,000 
4>Z5x  500  -  3.000 
4/6*  333-  2.000 
206  X  f66  -  t.OOO 

*(4.  Z  6£t 
f2.76    . 

9.94    - 
&.5Z   • 
7.1      « 
3.68    • 
4.26    • 
2.04    ' 
f.42  • 

"Zu's'so  >£*. 

374.65    . 

2O  /  .55    - 
249.90    • 
206.25    •' 
.'66.60    - 
/Z4.95   • 
03.30    - 
4f.65  • 

/         \f        \ 

/      A      \ 

/      /    \      \ 

/ 

/          \      \ 

/re£f7t     COY£e£0.                     \                        \ 

•                              ff/fea    Coveeeo.  \                \ 

f                                            \               \ 

/               /                       1                                            \ 

«^^l«^u^^il_^m^Jw^!^^L^^ 

FIG.  155. — Method  of  taking  stereoscopic  pictures. 


eyes,  or  65  millimeters.  If  d  is  the  interocular  distance,  a 
the  viewing  distance,  identical  with  the  focal  length  of  the 
lens  used,  and  A  the  altitude,  then  Z),  the  distance  between 
exposures,  is  given  by  the  relation — 


f\    t 


For  a  =  25  centimeters,       ~~9  approximately   J^,   or  the 


interval  between  exposures  must  be  a  quarter  the  altitude. 
With  a  50  centimeter  lens  this  becomes  Y%,  and  so  on.  These 
figures  show  the  fallacy  of  the  suggestion  sometimes  made 


STEREOSCOPIC  PHOTOGRAPHY 337 

that  we  take  stereoscopic  pictures  by  two  cameras  placed  one 
at  the  extremity  of  each  wing. 

When  lenses  of  more  than  25  centimeters  focal  length  are 
employed,  the  stereoscope  should  be  one  capable  of  throwing 
the  convergence  point  farther  away  than  the  customary 
25  centimeters.  In  the  simple  lens  type  of  instrument  we 
can  do  this  by  bringing  the  centers  of  the  lenses  closer 
together,  and  by  making  the  focus  agree  with  the  convergence 
point  by  adjustment  of  the  distance  between  lenses  and 
stereogram.  If  enlargements  are  used  they  should  be  treated 
in  all  respects  as  originals  made  by  lenses  of  the  greater  foci 
corresponding  to  the  scale  of  the  enlargement. 

When  all  the  conditions  are  covered,  the  appearance  pre- 
sented in  the  stereoscope  is  that  of  a  model  of  the  original  object 

at  a  distance  a,  and  ^  times  natural  size.  If  pictures  are 
made  at  exposure  intervals  less  than  those  indicated  for 
correct  relief,  they  show  insufficient  relief.  This  does 
not,  however,  give  an  unnatural  effect,  because  anything 
between  no  relief  and  "correct"  relief  appears  natural  with 
large  objects  which  are  not  ordinarily  seen  in  relief  by  eyes 
not  Brobdignagian.  Conversely,  stereograms  made  with  too 
large  exposure  intervals  show  exaggerated  relief.  Yet  this  is 
often  no  objection.  It  is  ingleed  rather  an  advantage  if  we 
wish  to  bring  objects  of  interest  to  notice.  Consequently, 
so  long  as  the  exaggeration  of  relief  is  not  offensive,  the 
permissible  limits  of  exposure  interval  are  pretty  large. 
Actually,  the  eye  tolerates  such  great  deviations  from  strictly 
normal  conditions  that  satisfactory  stereoscopic  effects  are 
obtained  for  pictures  viewed  at  very  different  distances  from 
the  focal  length  of  the  taking  lens,  and  with  the  axes  of  the 
eyes  parallel  or  even  diverging,  although  there  is  some  strain 
whenever  focus  and  convergence  points  differ.  On  the  whole, 
therefore,  it  may  be  said  that  the  conditions  above  laid  down 

22 


338         AIRPLANE  PHOTOGRAPHY 

for  correct  relief  are  only  a  normal,  to  be  approximated  as 
nearly  as  is  practicable. 

Having  established  the  correct  relation  of  taking  points 
for  stereos  the  next  problem  is  how  to  determine  these  when 
in  the  plane.  The  simplest  way  is  by  means  of  a  stereoscopic 
sight.  This  consists  essentially  of  two  lines  of  sight  (fixed  by 
beads,  crosses,  or  other  objects),  inclined  toward  each  other 
at  the  angle  determined  by  the  ratio  of  the  ocular  separation 
to  the  focal  length  of  the  lens.  If  the  back  sight  is  made  a 
single  bead  or  cross,  the  rest  of  the  stereo  sight  will  consist 
of  two  beads  or  crosses,  separated  from  each  other  by  the 
ocular  distance  of  65  millimeters,  and  distant  from  the  back 
sight  by  the  focal  length  of  the  lens  (Fig.  157).  The  first 
picture  is  taken  when  the  object  is  in  line  with  the  forward 
pointing  line  of  sight,  the  second  when  it  lies  along  the  back- 
ward pointing  one.  Like  other  sights,  the  stereoscopic  sight 
may  be  attached  either  to  the  camera,  or  if  this  is  fixed  in 
position,  to  any  convenient  part  of  the  plane.  A  very  simple 
sight  for  vertical  stereoscopic  photography  consists  of 
an  inverted  V  painted  on  the  side  of  the  fuselage,  so 
that  the  eye  can  be  placed  at  the  vertex  and  sighted  along 
either  leg. 

The  common  method  of  determining  the  space  between 
exposures  is  by  the  time  interval.  If  V  is  the  speed  of  the 
plane,  and  t  the  desired  time  interval,  we  have,  from  the 
last  equation— 

Z)  =dA 

V       aV 

If  A  =  2000  meters,  d  =  65  millimeters,  and  a  =  25  centimeters, 
and  if  the  plane  is  traveling  200  kilometers  per  hour,  the  time 
interval  must  be — 


STEREOSCOPIC  PHOTOGRAPHY 339 


At  1000  meters  altitude  the  interval  will  be  half  this,  and  so 
on  in  proportion.  If  the  pictures  are  taken  with  a  50  centi- 
meter focus  camera,  and  are  hence  to  be  viewed  at  50  centi- 
meters convergence  distance  instead  of  at  25,  the  time  will 
again  be  halved.  These  relations  are  clearly  shown  in  the 
diagram  (Fig.  156).  Here  the  left-hand  portion  shows  how 


HEI6HT IN  METERS 
1000  2000  3000  -*  00 


INTERVAL  IN  SECONDS 


FIG.  156. — Chart  for  calculating  intervals  between  exposures  for  stereoscopic  pictures. 

to  find  the  stereoscopic  base  line  at  each  altitude  for  each 
focal  length;  while  the  right-hand  portion  shows  how  to 
translate  this  into  time  interval  for  any  plane  velocity. 
The  Burchall  slide  rule  (Fig.  130)  shows  another  way  to 
arrange  these  data  in  form  for  rapid  calculation. 

Plates  used  for  stereoscopic  negatives  should  be  at  least 
twice  as  long  as  the  ocular  separation,  if  correct  relief  is 


340        AIRPLANE  PHOTOGRAPHY 

desired,  and  the  full  size  of  the  stereoscope  field  is  to  be 
utilized.  This  relation  follows  at  once  if  we  consider  that 
we  wish  to  cut  from  each  negative  a  rectangle  65  millimeters 
wide,  and  that  the  image  of  the  target  has  shifted  65  milli- 
meters between  exposures.  If  the  plate  is  larger  than  this 
there  is  opportunity  to  select  the  view,  or  to  pick  several. 
If  the  plate  is  smaller  the  elements  of  the  stereogram  must 
be  narrow  strips.  This,  however,  holds  only  for  contact  prints. 

The  ordinary  English  practice  in  making  stereo  negatives 
is  to  take  successive  pictures  with  an  overlap  of  60  to  75  per 
cent.  This  practice  is  probably  dictated  by  the  4X5  inch 
plate,  since  60  per  cent,  overlap  on  4  inches  means  a  separa- 
tion of  just  over  an  inch  and  a  half  instead  of  2%,  but  it 
leaves  2J^  inches  of  picture  common  to  the  two  negatives. 
With  %  overlap  the  common  portion  is  3  inches,  which  per- 
mits of  cutting  2^4  inch  prints,  and  allows  some  latitude  for 
irregular  motion  of  the  plane  or  for  chance  error  in  calcula- 
tion of  intervals.  Data  on  the  basis  of  %  overlaps  for  a 
4-inch  plate  are  shown  in  connection  w^ith  Fig.  155  which 
shows  hi  diagrammatic  form  the  variation  of  exposure  inter- 
val with  height,  together  with  other  points  of  interest. 

Elevation  Possible  to  Detect  in  Stereoscopic  Views. — 
Can  the  actual  difference  in  elevation  be  discovered  by  the 
use  of  stereoscopic  views?  An  approximate  idea  may  be 
obtained  from  the  following  considerations:  Suppose  we 
have  two  small  point-like  objects,  one  above  the  other,  such 
as  a  street  lamp  globe  and  the  base  of  the  lamp  pillar.  In  a 
view  taken  from  directly  overhead  these  will  be  superposed, 
and  so  will  not  be  capable  of  separation.  But,  as  the  point  of 
view  is  shifted  sideways,  the  two  objects  separate,  until  a  point 
is  reached  where  they  can  just  be  distinguished  as  double. 
When  this  condition  holds  for  either  picture  of  the  stereo- 
scopic pair  it  will  be  possible  to  obtain  stereoscopic  relief. 


STEREOSCOPIC   P  H  O  T  O  G  R  A  P  H  Y  341 

Now  the  separation  which  can  just  be  distinguished  is 
commonly  assumed  to  be  one  minute  of  arc.  This  angle 
corresponds  to  about  3-^0  tne  distance  from  the  eye  to  the 
object.  If  the  object  is  assumed  at  a  distance  a  from  the 
face,  and  on  a  line  with  one  of  the  eyes,  which  are 
separated  by  the  distance  d,  then  (all  angles  being  small) 
the  object  must  be  of  height  -j  times  the  horizontal  distance 

which  corresponds  to  one  minute.  For  25  centimeters'  view- 
ing-distance  this  quantity  is  about  4,  so  that  the  least  per- 
ceptible elevation  is  3/00  or  about  -g^.  The  stereogram 
having  been  made  under  conditions  giving  correct  relief, 
this  fraction  is  also  the  fraction  of  the  altitude  of  the  plane 
when  the  photograph  was  taken  which  may  be  detected. 
An  object  as  high  as  a  man  (6  feet)  should  be  visible  as  a 
projection  in  a  stereoscopic  view  taken  at  6X900  =  5400  feet. 
This  relation — -Q^-Q — holds  (irrespective  of  the  focal  length 
of  the  lens),  as  long  as  the  conditions  for  correct  relief 
are  maintained. 

Stereoscopic  Aerial  Cameras. — Cameras  for  aerial  stereo- 
scopic photography  need  in  no  way  differ  in  construction 
from  those  made  for  mapping  or  spotting,  provided  only 
they  permit  exposures  to  be  made  at  short  enough  intervals. 
The  addition  of  special  sights,  as  already  discussed,  consti- 
tutes the  only  real  difference  between  single  view  and  stereo- 
scopic aerial  cameras.  But  even  without  such  sights  ordinary 
aerial  cameras  are  applicable  to  stereo  work  by  the  usual 
procedure  of  determining  the  exposure  spacing  by  time. 

One  scheme  employed  for  taking  low  stereos,  where  the 
interval  is  only  two  or  three  seconds,  is  to  mount  two  cameras 
in  the  plane,  exposing  them  one  after  the  other  at  the  correct 
interval.  Another  method  which  has  been  tried  with  success 
is  the  use  of  a  double  focal-plane  shutter  in  a  single  lens 
camera  (Fig.  157).  The  two  shutters  are  side  by  side,  with 


342 


AIRPLANE  PHOTOGRAPHY 


their  slots  parallel  to  the  line  of  flight.  To  take  a  stereo 
negative  we  expose  first  the  shutter  nearer  the  tail  of  the 
plane,  and  then  the  other,  after  an  interval  which  can  be 
calculated  from  the  speed  and  altitude,  or,  better,  determined 
by  a  stereoscopic  sight.  The  two  views  are  thus  obtained 
on  a  single  plate.  Prints  from  these  negatives  are  transposed 


Fro.  157. — Aerial  hand  camera  fitted  with  two  complementary  shutter  slits   and   double  sight,  for 
stereoscopic  photography. 

right  and  left,  and,  if  the  prints  are  viewed  in  an  ordinary 
stereoscope,  have  to  be  cut  apart  and  transposed  for  mount- 
ing,, or  else  this  may  be  done  to  the  negatives. 

In  this  connection  attention  may  be  drawn  to  an  alterna- 
tive method  of  viewing  stereograms,  which  may  be  used  on 
transposed  prints — a  method  which  needs  no  instrument, 
and  so  has  sufficient  advantage  to  even  warrant  mounting 


STEREOSCOPIC  PHOTOGRAPHY  343 


ordinary  stereoscopic  pairs  in  the  transposed  position  for 
observation.  This  method  consists  in  crossing  the  optic 
axes,  in  the  fashion  illustrated  in  Fig.  158.  A  ringer  is  held 
in  front  of  the  face  in  such  a  position  that  the  left  stereogram 
element  and  the  finger  are  seen  in  line  by  the  right  eye;  the 
right  element  and  the  finger  by  the  left  eye.  The  proper 
position  is  found  by  alternately  closing  each  eye,  and  advanc- 
ing or  retracting  the  finger.  Then  both  eyes  are  opened  and 
converged  on  the  finger  tip,  which  is  thereupon  dropped, 
leaving  the  picture  standing  out  in  relief.  An  opportunity 
to  try  this  method  is  afforded  by  Fig.  159. 


FIG.  158. — Method  of  fusing  transposed  stereoscopic  images  by  crossing  the  optic  axes. 

Stereo  Obliques. — The  theory  of  making  oblique  stereo 
pictures  is  identical  with  that  of  other  stereos.  The  only 
problem  peculiar  to  obliques  is  that  of  making  the  exposures 
at  short  enough  intervals  apart.  This  problem  is  due  largely 
to  the  fact  that  oblique  views  are  ordinarily  taken  from  low 
altitudes,  for  the  purpose  of  "spotting"  particular  objects, 
rather  than  for  mapping  the  gross  features  of  an  extended 
area.  The  same  problem  of  how  to  secure  a  short  exposure 
interval  is  met  with  when  we  attempt  to  take  vertical  stereos 
from  a  low  altitude,  but  as  already  discussed,  it  is  much 
preferable  from  the  pictorial  standpoint  that  pictures  of 
definite  small  objectives  be  made  obliquely. 

Another  reason  for  taking  stereo  obliques  from  points  but 


344        AIRPLANE  PHOTOGRAPHY 

little  separated  is  of  some  interest  in  connection  with  the 
discussion  above  given  of  "correct"  and  "natural"  relief. 
When  the  relief  is  "correct"  the  object  appears,  as  already 
stated,  to  be  a  small  model  in  its  true  proportions,  standing 
at  the  convergence  distance.  When  the  eyes  are  converged 
to  a  small  object  25  to  50  centimeters  away  all  objects 
beyond  are  hopelessly  transposed  and  confused.  This  does 
not  happen  when  we  look  at  large  distant  objects,  since  their 
background  is  at  a  distance  effectively  but  little  beyond  them. 
As  a  result,  when  a  stereo  oblique  is  made  in  "correct"  relief 
of  such  an  object  as  the  Washington  monument  with  build- 
ings beyond,  the  confusion  of  the  background  presents  an 
appearance  entirely  contrary  to  our  visual  experience  with 
objects  as  large  as  the  neighboring  buildings  are  known  to 
be.  This  effect  may  be  avoided  by  choosing  a  uniform  back- 
ground such  as  grass,  or  by  taking  the  pictures  very  much 
closer  together,  at  the  expense  of  "correct"  but  at  a  gain 
in  "natural"  relief. 

Stereo  obliques  can  of  course  only  be  made  with  any 
facility  by  laterally  pointing  cameras.  From  the  calculations 
already  given  it  appears  that  a  "correct"  stereo  oblique  of 
an  object  500  meters  away  will  mean  exposures  only  two  or 
three  seconds  apart,  too  short  an  interval  for  any  of  the 
ordinary  plate-changing  and  shutter-setting  mechanisms; 
and  the  case  is  even  worse  should  less  relief  be  desired.  One 
solution  of  this  problem  has  been  the  use,  already  mentioned, 
of  two  cameras  mounted  together,  either  side  by  side  or  one 
over  the  other,  with  separate  shutter  releases.  Both  releases 
may  be  controlled  by  the  observer,  using  a  sight,  or  else 
pilot  and  observer  may  work  in  harmony  as  has  been  recom- 
mended in  the  English  service,  where  the  pilot  releases  one 
shutter  and  the  observer  counts  time  from  the  instant  he 
sees  the  first  shutter  unwind  and  releases  the  second. 


STEREOSCOPIC  PHOTOGRAPHY 345 

A  very  satisfactory  apparatus  for  the  taking  of  stereo 
obliques  consists  of  a  10-inch  focus  hand-held  camera  (Fig. 
157),  provided  with  a  two-aperture  focal-plane  shutter.  The 
right-hand  half  of  one  curtain  aperture  is  blocked  out,  the 
left-hand  half  of  the  other.  The  first  pressure  on  the  exposing 
lever  exposes  one-half  of  the  plate,  the  second  the  other.  A 


FIG.  159. — Oblique  stereogram  made  with  stereoscopic  aerial  camera  (Fig.  157).    To  be  viewed  by 
crossing  the  optic  axes  (Fig.  158). 

stereoscopic  sight  of  the  type  already  described  is  placed  on 
the  bottom.  To  make  an  oblique  stereo  negative  the  camera 
is  held  rigidly  by  resting  the  elbows  on  the  top  of  the  fusel- 
age and  the  first  exposure  is  made  when  the  object  comes  in 
line  with  the  rear  sight  and  the  leading  front  sight.  The  eye 
is  then  moved  so  as  to  look  along  the  line  of  the  rear  sight 
and  the  following  front  sight,  and  when  the  object  is  again 
in  alinement  the  second  pressure  is  given  the  exposing  lever. 


346        AIRPLANE  PHOTOGRAPHY 

Fig.  159  shows  a  stereo  oblique  made  by  this  camera.  The 
elements  are  transposed  right  and  left,  and  the  stereogram 
may  be  viewed  by  crossing  the  optic  axes  as  shown  in  Fig. 
158,  or  the  two  pictures  may  be  cut  apart  and  remounted. 

The  Mounting  of  Aerial  Stereograms. — The  first  step  in 
making  the  printed  stereogram  is  to  select  two  pictures  taken 
on  the  same  scale,  but  from  slightly  different  positions. 
These  may  be  two  chosen  from  a  collection  made  for  other 
purposes,  or  else  a  pair  taken  at  distances  calculated  to  fit 
them  for  stereoscopic  use.  The  next  step  is  to  mark  the 
center  of  each  picture,  either  with  easily  removed  chalk  or 
witn  a  pin  point.  They  are  then  superposed,  and  afterward 
carefully  moved  apart  by  a  motion  parallel  to  the  line  joining 
their  centers  when  superposed.  The  final  step  before  mount- 
ing is  to  mark  out  and  cut  the  two  elements,  their  bases  being 
parallel  to  the  line  of  centers,  their  horizontal  length  the  dis- 
tance between  the  optic  axes  of  the  stereoscope  (or  as  near 
this  as  the  size  of  the  prints  will  permit).  They  are  then 
mounted  on  a  card,  with  their  centers  separated  by  approxi- 
mately 65  millimeters.  The  right-hand  view  is  the  one  show- 
ing more  of  the  right-hand  side  of  objects,  and  vice  versa. 
This  process  of  arranging,  cutting,  and  mounting  is  shown 
clearly  in  Fig.  160.  In  this  case  the  stereoscopic  elements 
lie  symmetrically  about  the  line  joining  the  centers  of  the 
original  prints.  This  is  not  necessary,  as  they  may  be  selected 
from  above  or  below  this  line  so  long  as  their  bases  are  par- 
allel to  it.  A  simplification  of  this  method  consists  in  super- 
posing the  two  prints,  laying  over  them  a  square  of  glass  of 
the  size  to  which  they  are  to  be  cut,  then  turning  it  so  that  a 
side  is  parallel  to  the  line  of  centers,  and  cutting  around  it 
through  both  prints  with  a  sharp  knife.  The  principle  and 
results  are  of  course  the  same  with  both  methods. 

If  large  numbers  of  stereoscopic  prints  are  required  it  is 


348        AIRPLANE  PHOTOGRAPHY 

necessary,  for  economy  of  time,  either  to  photograph  a  fin- 
ished stereogram  and  make  prints  from  this  copy  negative, 
or  to  set  up  special  printing  machines.  Under  the  general 
discussion  of  printing  devices  a  stereoscopic  printer  is 
described  (the  Richard)  in  which  the  two  negatives  are 
placed  so  that  stereo  prints  can  be  got  by  two  successive 
printings  on  one  sheet  of  paper. 

Uses  of  Stereoscopic  Aerial  Views. — Attention  has 
already  been  called  to  the  characteristic  flatness  of  the  aerial 
view.  Neither  the  picture  on  the  retina  nor  that  on  the 
photographic  plate  affords  any  adequate  idea  of  hills  anoT 
hollows.  Unless  shadows  are  well  defined,  small  local  ele- 
vations and  depressions  cannot  be  distinguished  from  mere 
difference  in  color  or  marking.  Even  in  the  presence  of 
shadows  it  is  often  only  by  close  study  that  differences  of 
contour  are  noticeable.  But  with  stereoscopic  views  these 
features  stand  out  in  a  striking  manner.  Taking  our  illus- 
trations from  military  sources,  we  may  note  the  use  of  stereo- 
scopic pictures  to  detect  undulations  of  ground  in  front  of 
trenches  (Fig.  161).  They  reveal  the  hillocks,  pits,  small 
quarries,  streams  flowing  behind  high  banks,  and  other 
features  which  make  the  attack  hard  or  easy.  Commanding 
positions  are  shown,  the  boundaries  of  areas  exposed  to 
machine-gun  fire,  and  the  defilades  where  the  attackers  may 
pause  to  reform.  Concrete  "pill  boxes"  are  located  in  the 
midst  of  shell  holes  of  the  same  size  and  outline,  and  can  be 
differentiated  from  them. 

Railway  or  road  embankments  and  cuts  can  be  detected 
and  studied  to  extraordinary  advantage  in  stereoscopic  pic- 
tures. Thus  what  appears  to  be  a  mine  crater  on  a  level 
road,  easily  driven  around,  may  be  a  gap  blown  in  an 
embankment,  a  serious  obstacle  indeed.  Bridges,  observa- 
tion towers  and  other  elevated  structures  jump  into  view  in 


STEREOSCOPIC  PHOTOGRAPHY  349 


FIG.  161. — Typical  stereogram  of  military  detail.    Fuse  by  looking  at  a  distant  object  over  the  top 
of  the  page,  and  quickly  dropping  the  eyes  to  the  print. 

the  stereoscope  when  often  they  have  entirely  eluded  notice 
in  the  ordinary  flat  picture.  Once  presented  in  relief,  camou- 
flaged buildings  or  gun  emplacements,  however  carefully 
painted,  are  ridiculously  easy  to  pick  out. 

Practical  peace-time  applications  of  stereoscopic  views 
can  easily  be  foreseen  following  the  lines  of  war  experience. 
Such,  for  instance,  would  be  the  study  of  proposed  railway  or 
canal  routes.  A  series  of  stereograms  would  obviate  the 
necessity  of  contour  surveys,  at  least  until  the  exact  route 
was  picked  and  construction  work  ready  to  start. 

Apart  from  their  utilitarian  side,  however,  stereoscopic 
views  have  very  great  pictorial  merit.  Stereoscopic  pictures 
of  cathedrals,  public  and  other  large  buildings,  have  often 
great  beauty,  and  afford  opportunities  for  the  study  of  form 
given  by  no  other  kind  of  representation,  short  of  expensive 
scale  models.  They  may  very  well  lead  in  the  near  future  to  a 
revival  of  the  popularity  of  the  stereoscope. 

Impression  of  Relief  Produced  by  Motion. — An  appear- 


350         AIRPLANE  PHOTOGRAPHY 

ance  of  solidity  can  be  obtained  in  moving  pictures  by  the 
simple  expedient  of  slowly  moving  the  camera  laterally  as 
the  pictures  are  taken.  As  an  illustration,  if  the  moving 
picture  camera  is  carried  on  a  boat  while  structures  on  the 
shore  are  photographed,  when  these  are  projected  on  the 
screen  they  appear  in  relief,  due  to  the  relative  motion  of 
foreground  and  background.  As  relief  of  this  sort  is  not 
dependent  on  the  use  of  the  two  eyes,  it  demands  no  special 
viewing  apparatus.  This  idea  has  been  utilized  to  a  limited 
extent  in  ordinary  moving  picture  photography  by  intro- 
ducing a  slow  to-and-fro  motion  of  the  camera,  but  this  can 
hardly  be  considered  satisfactory,  since  this  motion  is  so 
obviously  unnatural. 

In  moving  pictures  made  from  the  aiplane  the  normal 
rapid  motion  of  the  point  of  view  is  ideal  for  the  production 
of  the  impression  of  relief  in  the  manner  just  described.  For 
instance,  in  moving  pictures  of  a  city  made  from  a  low  flying 
plane,  the  skyscrapers  and  spires  as  they  sweep  past  stand 
forth  from  their  more  slowly  moving  background  in  bold  and 
satisfying  solidity.  In  fact,  such  pictures  probably  constitute 
the  most  satisfactory  solution  yet  found  of  the  vexing  prob- 
lem of  "stereoscopic"  projection.  No  better  medium  can  be 
imagined  for  the  travel  lecturer  to  introduce  his  audience  to  a 
foreign  city  than  to  throw  upon  his  screen  a  film  made  in  a 
plane  approaching  from  afar  and  then  circling  the  archi- 
tectural landmarks  at  low  altitudes. 


CHAPTER  XXIX 

THE  INTERPRETATION  OF  AERIAL 
PHOTOGRAPHS 

Oblique  aerial  photographs  if  on  a  large,  enough  scale  are 
even  easier  to  interpret  than  are  ordinary  photographs  taken 
from  the  ground,  since  they  practically  preserve  the  usual 
view,  and  add  to  it  the  essentials  of  a  plan.  With  verticals, 
however,  this  is  far  from  the  case.  In  them  all  natural  objects 
present  an  appearance  quite  foreign  to  the  ordinary  mortal's 
previous  experience  of  them.  This  may  be  easily  demon- 
strated by  taking  any  aerial  view  containing  a  fair  amount 
of  detail  and  trying  systematically  to  identify  each  object. 
A  necessary  preliminary  to  doing  this  accurately  is  acquaint- 
ance with  and  study  of  the  ground  photographed,  or  of 
similar  regions,  and  of  objects  of  the  same  character  as 
those  likely  to  be  included. 

The  interpretation  of  military  aerial  photographs  is  of 
such  importance,  and  has  become  such  an  art,  that  it  is  the 
function  of  special  departments  of  the  intelligence  service. 
Extended  courses  in  the  subject  are  now  given  in  military 
schools.  This  instruction  must  cover  more  than  the  inter- 
pretation of  aerial  photographs  as  such.  General  military 
knowledge  is  essential,  so  that  not  only  may  photographed 
objects  be  recognized,  but  the  significance  of  their  appear- 
ance be  realized.  Whether  attack  or  retreat  is  indicated; 
whether  a  long  range  bombardment  is  in  preparation,  or  a 
mere  strengthening  of  local  defences. 

The  natural  difficulties  of  interpreting  aerial  views  are 
enormously  increased  by  the  unfamiliar  nature  and  fre- 
quently changed  character  of  the  military  structures,  and 

351 


352        AIRPLANE  PHOTOGRAPHY 

particularly  by  the  attempts  made  to  conceal  these  from 
aerial  observation  by  selection  of  surroundings  and  by  cam- 
ouflage. The  small  scale  of  the  photographs,  in  which  a 
machine  gun  shows  as  a  mere  pin  point,  adds  to  the  uncer- 
tainty, with  the  net  result  of  making  interpretation  a  task 
of  minute  study  and  deduction  worthy  of  a  Sherlock  Holmes. 

Little  detailed  information  on  interpretation  can  be 
profitably  written  in  a  general  treatise,  partly  because  the 
illustrations  available  are  of  a  highly  technical  military  char- 
acter, partly  because  original  photographs  instead  of  half- 
tone reproductions  are  practically  imperative  for  purposes 
of  study.  Nevertheless  some  general  instructions,  applicable 
to  any  problem  of  interpretation,  may  be  given,  as  well  as  a 
few  illustrations,  drawn  from  military  sources,  which  will 
serve  to  show  the  detective  skill  necessary. 

First  of  all  it  is  important  that  the  print  or  transparency 
be  held  in  the  right  position.  The  shadows  must  always  fall 
toward  the  observer;  otherwise,  reliefs  will  appear  as  hollows 
and  hollows  will  show  as  hills.  The  reason  for  this  is  that  the 
body  ordinarily  acts  as  a  shield,  preventing  the  formation  of 
shadows  except  by  light  falling  toward  the  beholder.  Thus 
in  Fig.  162  the  slag  heap  looks  like  a  quarry  when  the 
shadows  fall  away  from  one.  The  necessity  for  proper  direc- 
tion of  shadows  is,  it  may  be  noted,  in  conflict  with  the 
ordinary  convention  for  the  orientation  of  maps — at  least  in 
the  northern  hemisphere.  A  city  map,  made  by  sunlight 
falling  from  the  south,  presents  its  shadows  as  falling  away 
from  the  observer,  when  it  is  mounted  with  its  north  point 
at  the  top,  as  is  customary.  As  a  consequence  buildings  in 
aerial  photographic  mosaics  of  cities  occasionally  look 
sunken  instead  of  standing  out. 

The  relation  between  the  shape  of  the  shadow  and  the 
object  casting  it  must  be  well  learned.  This  is  a  part  of  the 


354        AIRPLANE  PHOTOGRAPHY 


'B         C 


D     E 


B        C 

=3J 
A  D     £ 


A  £ 

C  D  | 

£ 

B'"C      D 


L/GHT  &  SHADE  ON 

FIG.  163. — Guide  to  interpretation  of  trench  details. 


training  of  every  architectural  draftsman,  but  the  appear- 
ance of  shadows  from  above  has  not  heretofore  been  a  matter 
of  importance.  The  difference  between  high  and  low 


INTERPRETATION 


355 


f 


LIGHT  4MD  SHADE  /A/  SHELL-MOLES 


\ 


FNTfiANC£  TO 
Dt/6  Ol/r 


ORGAN/Z££)     SHELL  -  HOL£ 


FIG.  164. — Guide  to  interpretation  of  shell  holes  and  other  pits. 

trenches,  between  cuttings  and  embankments,  between  shell 
holes,  occupied  or  unoccupied,  and  "pill  boxes,"  must  be 
detected  largely  from  the  character  of  the  shadows.  Which 


356         AIRPLANE  PHOTOGRAPHY 

elevations  and  depressions  are  of  military  and  which  of 
merely  accessory  nature,  whether  this  black  dot  is  a  machine 
gun  or  a  signaling  device,  whether  that  dark  spot  is  an  active 
gun  port  or  an  abandoned  one — these  are  all  matters  of 
shadow  and  of  light  and  shade  study.  Several  illustrations 
of  these  points  appear  in  Figs.  163,  164  and  165. 

Shadows  may  be  used  to  get  exact  information  as  to 
directions  and  magnitudes.    If  we  know  the  time  of  day  at 


A.A.  6un  emfifacemenf 

with  concrete 
_,  centre  fiil/ars 

FIG.  165. — Illustrating  the  importance  of  distinguishing  between  objects  of  similar  appearance  but 
different  military  importance. 

which  a  picture  is  taken,  the  direction  of  the  shadows  will 
give  the  points  of  the  compass.  A  chart  for  doing  this  is 
shown  in  Fig.  166.  The  length  of  a  shadow  is  a  measure  of 
the  height  of  the  object  casting  it,  and  the  exact  relation 
between  the  two  dimensions  is  determined  by  the  day  and 
hour.  Fig.  167  embodies  in  chart  form  the  values  of  this 
relationship  for  all  times  of  the  year  and  day,  while  Fig.  168 
shows  the  kind  of  picture  in  which  shadow  data  could  be 
utilized  to  great  profit. 

Minute  changes,  both  in  light  and  shade  and  in  position, 
must  be  watched  for  with  great  care.     Naturally  growing 


INTERPRETATION  357 

foliage  and  the  cut  branches  used  for  camouflage  differ  in 
color  progressively  with  the  drying  up  of  the  leaves.  Hence 
a  mere  spot  of  lighter  tone  in  a  picture  of  a  forest,  especially 


•IX 


XI 
XII 


III 
III 


15 


FIG.  166.— Location  of  true  north  from  direction  of  shadows.  Place  the  dial  on  the  photo- 
graph, the  hour  line  corresponding  to  the  time  it  was  taken  being  pointed  in  the  direction  of 
the  shadows.  North  lies  between  the  two  arrows,  the  exact  direction  being  obtained  by  joining  the 
center  of  the  dial  to  the  point  on  the  figure  of  eight  corresponding  to  the  date  on  which  the  picture  was 
taken.  (Numbers  on  figure  of  eight  represent  the  1st  of  the  month). 

if  the  picture  is  taken  through  a  deep  filter,  becomes  instant 
object  for  suspicion.  The  complete  study  of  any  position 
calls  for  photographs  of  all  kinds — verticals,  obliques,  and 


AIRPLANE  PHOTOGRAPHY 


stereos.  Stereoscopic  views  are  the  worst  foe  to  camouflage. 
A  bridge  painted  to  look  like  the  river  beneath  is  labor 
thrown  away  if  the  stereo  shows  it  to  be  a  good  ten  feet 
above  the  real  river! 

December 

November 

October 


December 


FIG.  167. — Length  of  shadow  of  object  one  meter  high,  at  different  times  of  the  day  and  year,  for 

latitude  of  Paris. 

A  few  illustrations  of  the  more  ordinary  and  obvious 
objects  whose  detection  is  the  subject  of  aerial  photography 
are  shown  in  accompanying  figures.  Fig.  169  pictures  a 
typical  trench  system,  with  barbed  wire.  The  trenches  show 
as  narrow  castellated  lines,  from  which  run  the  zigzag  lines 
of  communicating^trenches,  saps,  and  listening  posts.  The 


INTERPRETATION 


359 


360 


AIRPLANE  PHOTOGRAPHY 


minute  pockmarks  behind  the  main  trench  lines  are  shell 
holes  and  machine  gun  pits.  The  barbed  wire  shows  as 
double  and  triple  gray  bands,  intricately  criss-crossed  at 


FIG.  169. — Typical    trench   photograph   showing   first   and   second   lines,   communicating   trencher, 
listening  posts,  machine  gun  emplacements,  and  barbed  wire. 

strategic  points.  Another  form  of  defence,  intended  for  the 
same  purpose  as  the  barbed  wire  of  the  western  front,  is  that 
furnished  by  overthrown  trees  in  forest  regions.  Fig.  170 
reveals  a  mountain  fortress  surrounded  by  a  zone  of  felled 


INTERPRETATION 


361 


trees,  and  indicates  in  striking  manner  the  value  of  the  infor- 
mation a  single  aerial  photograph  may  furnish  to  an  attack- 
ing force.  Fig.  123  shows  on  a  comparatively  large  scale 
opposing  trench  systems  in  which  a  natural  obstacle — a 
river — separates  the  adversaries.  Nicks  and  dots  indicate 
machine  guns  to  the  skilled  eye,  and  several  rectangular 


FIG.  170. — A  mountain  fort  surrounded  by  felled  timber. 

structures  are  revealed  as  concrete  buildings  which  have 
survived  unscathed  the  shell  fire  which  has  obliterated,  and 
caused  to  be  rebuilt,  nearly  every  other  element  of  the 
trench  system. 

Isolated  battery  emplacements  (Fig.  171)  must  be  care- 
fully studied  to  learn  if  they  are  in  use.  The  chief  indication 
is  given  by  the  paths  the  men  make  in  going  and  coming; 
these  show  as  fine  light  lines,  obliterated  by  growing  vege- 


362 


AIRPLANE  PHOTOGRAPHY 


Ficu  171. — Three  stages  in  the  life  of  a  battery. 


INTERPRETATION  363 

tation  if  long  disused.  Another  indication  is  the  blast  marks 
in  front  of  the  gun  muzzles;  occasionally  the  sensitive  plate 
will  catch  the  actual  puff  of  smoke  as  the  gun  is  discharged. 

Railways  of  various  gauges  show  as  thin  lines,  crossed 
by  ties,  and  exhibiting  the  characteristic  curves  and  switches. 
They  are  particularly  important  to  detect  because  they 
naturally  lead  to  guns  or  supplies  of  importance.  Abandoned 
railways  from  which  the  rails  and  ties  have  been  removed 
leave  their  marks  on  the  ground  and  must  be  carefully 
distinguished  from  lines  in  actual  operation. 

Aviation  fields  are  easily  recognized  by  the  hangars, 
often  with  "funk  hole"  trenches  alongside  for  the  men  to 
take  shelter  in  during  air  raids  (Fig.  172).  Other  character- 
istic features  are  the  "T"  which  shows  the  direction  of  the 
wind  to  the  returning  pilot,  and  of  course  the  planes  them- 
selves, standing  on  the  ground.  But  the  field  may  be  inac- 
tive, and  the  planes  merely  canvas  dummies,  so  that  to 
pierce  the  disguise,  all  paths,  ruts,  and  other  indications  of 
activity  must  be  minutely  studied. 

Overhead  telegraph  and  telephone  lines  are  revealed 
when  new  by  a  series  of  light  points  (Fig.  174),  where  the 
posts  have  been  erected  in  the  fresh  turned  earth.  Later, 
when  the  fields  through  which  they  pass  are  cultivated,  the 
post  bases  show  as  islands  left  unturned  by  the  plow.  In 
winter  the  wires  reveal  their  position  by  black  lines  in  the 
snow  caused  by  drippings.  Buried  cables  are  indicated 
while  building  by  their  trenches,  and  for  some  time  after- 
ward by  the  comparatively  straight  line  of  disturbed  earth. 

Just  as  the  detective  of  classic  story  makes  full  use  of 
freshly  fallen  snow  to  identify  the  footprints  of  the  criminal, 
so  does  the  aerial  photographer  utilize  a  snowfall  to  pierce 
the  enemy's  attempts  at  deception.  Tracks  in  the  snow 
show  which  trenches  or  batteries  are  in  actual  use.  Melting 


364 


AIRPLANE  PHOTOGRAPHY 


FIG.  172. — Aviation  field,  showing  hangars,  planes,  landing  "T"  and  refuge  trench. 

of  the  snow  in  certain  places  may  mean  fires  in  dugouts 
beneath.  Black  smudges  in  front  of  trench  walls  show  where 
guns  are  active.  Guns,  wire  and  other  objects,  however 


INTERPRETATION 


365 


ROAD 


FIG.  174. — A  fully  interpreted  aerial  photograph. 


INTERPRETATION  367 

carefully  painted  to  match  the  gray-green  earth,  stand  out 
in  violent  contrast  to  this  new  white  background  (Fig.  173). 
After  the  aerial  photograph  has  been  interpreted  the 
results  of  the  interpretation  must  be  made  available  to  the 
artilleryman  or  the  attacking  infantryman.  This  may  be 
done  by  legends  marked  directly  on  the  photograph.  Another 
method  is  to  mount  over  the  photograph  a  thin  tissue  paper 
or  oilskin  leaf,  with  the  interpretation  marked  on  it.  A  yet 
more  elegant  method  consists  in  outlining  all  the  chief  fea- 
tures of  the  photograph  in  ink,  writing  in  the  points  of 
importance  in  interpretation,  and  then  bleaching  out  the 
photograph  with  potassium  permanganate  solution.  Photo- 
graphic copies  of  the  resultant  line  drawing  are  then  mounted 
side  by  side  with  the  original  photograph.  Fig.  174,  which 
shows  a  fully  interpreted  photograph,  is  an  example  of  this 
kind  of  mounting. 


CHAPTER  XXX 
NAVAL  AERIAL  PHOTOGRAPHY 

The  problems  of  naval  aerial  photography  are  qiute 
different  from  those  of  military  aerial  work,  and  on  the  whole 
they  are  more  simple.  At  the  same  time,  photography  has 
played  a  considerably  less  important  part  in  naval  aerial 
warfare  than  in  land  operations.  Photography  as  a  necessary 
preparation  for  attack  has  not  figured  in  naval  practice 
nearly  so  much  as  have  the  record  and  instruction  aspects. 
To  some  extent  this  is  due  to  the  nature  of  the  naval  opera- 
tions in  the  Great  War,  to  some  extent  to  the  limitations  of 
ceiling  and  cruising  radius  of  the  naval  aircraft. 

A  photographic  reconnaissance,  preceding  and  following 
a  bombardment  of  shore  batteries;  a  photographic  record  of 
the  ships  at  anchor,  as  at  Santiago;  a  photograph  of  the  forts 
defending  a  channel,  as  at  Manila;  photographs,  quickly 
developed  and  printed,  of  an  approaching  fleet — all  these 
are  possibilities  of  great  usefulness  in  naval  warfare  between 
contestants  both  of  whom  "come  out"  and  carry  the  strug- 
gle to  the  enemy's  gates.  But  in  the  recent  war  the  use  of  the 
submarine,  operations  under  cover  of  fog,  the  striving  for 
"low  visibility,"  and  the  considerable  distances  to  be 
traversed  to  reach  the  enemy  lairs,  have  conspired  to  limit 
the  development  of  photography  as  a  major  aid  to  naval 
combat.  Probably  when  the  whole  history  of  the  conflict 
is  told  we  shall  learn  that  the  Zeppelins  which  cruised  over 
the  North  Sea,  keeping  the  Allied  fleet  under  observation, 
had  a  regular  routine  of  photographic  work.  In  the  Italian 
zone,  where  much  of  the  enemy  territory  and  several  impor- 
tant naval  centers  lay  at  only  short  distances  over  the  Adri- 


NAVAL  PHOTOGRAPHY 


369 


atic,  the  naval  photographic  service  more  nearly  rivalled 
that  of  the  army  than  in  the  English,  French  and  American 
zone  of  activity  in  the  Channel  and  North  Sea. 

The  majority  of  the  photographs  made  in  the  British 
service  were  obliques,  taken  by  short  focus  (6  to  10  inch) 


FIG.  175. — A  lighthouse,  as  the  naval  flier  sees  it. 


hand-held  cameras.  This  type  was  employed  partly  because 
of  difficulties  to  be  noted  presently,  in  using  other  forms  of 
cameras,  but  more  especially  because  such  pictures  sufficed 
for  the  kind  of  information  desired.  A  hand-held  camera 
formed  part  of  the  outfit  of  each  flying  boat  and  diri- 
gible, but,  unlike  land  reconnaissance,  planes  ascending 
primarily  for  picture  taking  were  unknown  in  their  naval 
service.  Usually  no  photographic  objective  was  predeter- 

24 


370 


AIRPLANE  PHOTOGRAPHY 


mined — photographs  were  made  only  if  objects  of  interest 
were  come  upon.  Mapping  also  formed  no  part  of  the  sea- 
plane's work.  Four  plates  would  be  carried,  instead  of  as 
many  dozens  in  the  land  machine,  and  often  these  would 
come  back  unexposed.  There  were  of  course  some  photo- 
graphic flights  planned  out  beforehand,  for  the  purpose  of 
photographing  lighthouses  and  other  landmarks  whose 


FIG.  176. — A  threatened  submarine  attack.     Throwing  out  a  smoke  screen  to  protect  a  convoy. 

British  official  photograph. 

appearance  from  the  air  should  be  known  to  the  naval  avia- 
tor (Fig.  175).  Among  the  accidental  and  record  types  of 
photograph  come  convoys  (Fig.  176),  whose  composition  and 
arrangement  were  made  a  matter  of  record,  particularly  if 
any  ship  was  out  of  its  assigned  position.  Photographs  of 
oil  spots  on  the  sea  surface,  or  other  results  of  bomb  dropping, 
were  necessary  evidence  to  establish  the  sinking  of  a  subma- 
rine (Fig.  179).  Pictures  of  all  types  of  ships  friendly, 


NAVAL  PHOTOGRAPHY 


371 


neutral,  and  where  possible,  enemy — were  a  much  needed 
part  of  naval  equipment,  in  particular  pictures  of  friendly 
destroyers  and  submarines,  which  should  not  be  bombed 
by  mistake.  For  safe  navigation  it  was  essential  to  have 
photographs  of  uncharted  wrecks  (Fig.  181),  of  buoys  out 
of  place  and  of  ships  failing  to  return  signals  or  otherwise 


FIG.  177. — Submarine  coming  to  the  surface. 
U.  S.  Naval  'Air  Service  photograph 

to  comply  with  rules.     The  great  majority  of  the  pictures 
were  taken  from  altitudes  of  not  more  than  300  meters. 

Hand-held  cameras  for  naval  work  have  practically  the 
same  design  as  those  for  land  work.  In  view  of  the  smaller 
number  of  pictures  taken  on  naval  trips,  and  the  consequent 
absence  of  any  need  for  great  speed  in  changing  plates,  the 
ordinary  two-plate  dark  slide  has  been  found  satisfactory  in 


372         AIRPLANE  PHOTOGRAPHY 


FIG.  178. — Dropping  depth  bombs. 


FIG.  179. — The  submarine  destroyed.    Destroyer  on  tell-tale  oil  patch. 
British  official  photographs. 


NAVAL  PHOTOGRAPHY 


373 


the  English  service.  But  these  are  much  less  convenient 
than  the  bag  magazines  used  in  the  U.  S.  Naval  hand  camera 
(Fig.  31).  The  sights  on  the  naval  hand  camera  are  prefer- 
ably of  the  rectangular,  field  indicating  type,  especially  useful 


FIG.  180. — A  convoy  at  anchor  in  port. 

in  photographing  extended  objects  such  as  convoys.  As  the 
flying  boat  travels  comparatively  slow,  it  is  easy  for  the 
observer  to  stand  up  to  take  pictures,  and  the  sight  is  con- 
veniently placed  on  top.  But  if  held  out  over  the  side  for 
verticals  the  sight  must  be  on  the  bottom.  Rectangular 
sights  in  both  positions  are  provided  in  the  English  camera 


374        AIRPLANE  PHOTOGRAPHY 

(Fig.  186) .  Naval  cameras  should  be  immune  from  moisture, 
which  means  doing  away  with  all  wooden  slides  or  grooves. 
A  praiseworthy  practice  is  to  carry  the  camera  in  a  water- 
proof bag. 

Cameras  other  than  of  the  hand-held  form  have  been 
little  used  in  sea  planes,  owing  to  the  difficulties  of  installa- 


FlG.  181. — Airplane  photography  as  an  aid  to  salvaging.     Position  of'  wrecked  merchantman  twelve 

fathoms  down  revealed  by  photograph  from  the  air. 

Photograph  by  British  Air  Service. 

tion.  The  hydro-airplane,  consisting  of  an  ordinary  airplane 
fuselage  mounted  on  two  pontoons  (Fig.  182),  can  carry  the 
same  kind  of  photographic  equipment  as  the  land  machine. 
But  if  it  has  a  single  central  pontoon  this  is  not  feasible. 
The  hydro-airplane  is,  however,  largely  superseded  by  the 
flying  boat  (Fig.  183),  whose  fuselage,  of  boat  form,  rests 
directly  on  the  water.  In  this  type  of  sea  plane,  views  taken 


NAVAL  PHOTOGRAPHY 


375 


FIG.  182.— A  sea  plane. 


FIG.  183.— A  flying  boat. 


vertically  downward  are  not  easy  to  make.  In  the  larger 
flying  boats  the  hull  projects  out  horizontally  a  matter  of 
several  feet  beyond  the  side  of  the  cockpit.  An  ordinary 


376        AIRPLANE  PHOTOGRAPHY 


1*10.  184. — A  dirigible  or  "blimp" — possibly  the  photographic  aircraft  of  the  future. 


FIG.  185. — English  "Type  18"  hand  camera  on  bracket  for  exposing  through  side  window  of  flying  boat. 
British  official  photograph. 


NAVAL  PHOTOGRAPHY 


377 


outboard  mounting  is  therefore  out  of  the  question.  The 
camera  must  either  be  held  out  at  arm's  length  or  else 
mounted  on  a  long  bracket  (Fig.  186).  The  usual  place  for 
carrying  the  camera  is  in  the  front  cockpit  with  its  magnifi- 
cent all-round  view.  Obliques  can,  too,  be  taken  in  great 
comfort  from  the  side  windows  behind  the  wings,  as  shown 


FIG.  186. — Camera  mounted  in  bracket  from  forward  cockpit  of  flying  boat. 
British  official  photograph. 

in  Fig.  185.  The  possibility  of  cutting  a  hole  in  the  bottom 
of  a  flying  boat  to  take  care  of  a  vertical  camera  is  not 
entertained  in  British  and  American  naval  cricles.  Never- 
theless it  is  the  regular  practice  in  the  Italian  service,  with 
their  small  high  ceilinged  flying  boats.  In  them  a  round  hole 
is  cut  in  the  floor,  stopped  with  a  plug  and  rubber  gasket. 
After  the  boat  rises  into  the  air  the  hole  is  opened,  and  the 


378 


AIRPLANE  PHOTOGRAPHY 


regulation  Italian  camera  is  set  securely  in  a  frame  on  the 
floor  over  the  hole  (Fig.  187).  Photographs  are  taken  to 
the  capacity  of  the  camera,  and  if  it  is  desired  another  camera 
is  put  in  its  place,  till  all  its  plates  have  been  exposed,  and 
then  even  a  third.  Before  coming  down  the  hole  must 
of  course  be  closed  again.  Sliding  doors  have  been  de- 
signed to  close  this  aperture,  but  have  not  proved  sufficiently 


Postazjpne  della  mscchma  fofo^rajtcci  sujji  idrovoldnti 


M 


«?r-i-,      i 


FIG.  187. — Italian  flying  boat  with  camera  mounted  on  the  floor. 

water-tight,  although  such  a  device  could  undoubtedly  be 
worked  out. 

With  its  space  for  five  or  more  passengers,  and  with  its 
low  speed,  the  modern  flying  boat  affords  an  excellent  craft 
for  photographic  work.  There  is  ample  room  for  any  size 
of  camera,  and  for  any  style  of  mounting,  if  we  assume  that 
there  is  no  objection  to  an  opening  in  the  bottom.  The  low 
ceiling  of  these  ships,  however,  prevents  their  use  for  certain 
forms  of  aerial  photography  which  should  be  of  the  greatest 
importance.  Operations  against  shore  stations — harbors, 


NAVAL  PHOTOGRAPHY  379 

docks,  shipyards,  ships  at  anchor,  and  fortifications — cannot 
be  undertaken  for  fear  of  anti-aircraft  guns  and  hostile  land 
planes.  The  solution  of  the  problem  of  carrying  and  launch- 
ing fast  high  flying  planes  from  ships  will  immediately  extend 
the  usefulness  of  aerial  photography  to  coastal  work.  In 
the  recent  war,  such  of  this  as  was  done,  along  the  Belgian 
coast — the  shore  batteries,  and  the  results  of  naval  operations 
at  Zeebrugge  and  Ostend — was  done  by  land  planes  from 
territory  held  by  the  Allies.  The  photographic  equipment  of 
sea  planes  of  the  type  suggested  will  of  course  present  special 
problems,  but  the  apparatus  used  will  be  apt  to  approximate 
closely  to  that  of  the  land  planes. 


VII 

THE  FUTURE  OF  AERIAL  PHOTOGRAPHY 


CHAPTER  XXXI 

FUTURE   DEVELOPMENTS   IN  APPARATUS 
AND   METHODS 

Prophecy  is  an  undertaking  that  always  involves  risk. 
The  prophet's  guess  of  what  the  future  will  bring  forth  is 
based  only  on  the  tendencies  of  the  past,  the  most  urgent 
needs  of  the  present,  and  the  activity  of  his  imagination. 
He  may  easily — and  he  usually  does — entirely  overlook 
certain  possibilities  which  may  arise  apparently  from  no- 
where and  which  profoundly  affect  the  whole  trend  of  devel- 
opment. Conditions  which  dominate  at  the  present  time — 
such  as  military  necessity — may  happily  drop  into  the  back- 
ground and  free  the  science  from  some  of  its  severest  restric- 
tions. With  this  caution,  some  future  possibilities  in  appa- 
ratus and  methods  may  be  presented  along  the  lines  already 
used  in  discussing  the  present  status  of  aerial  photography. 
Lenses. — Prom  the  military  standpoint  the  next  steps 
in  lens  design  would  be  toward  telephoto  lenses  on  the  one 
hand,  and  on  the  other  toward  lenses  of  short  focus  and  wide 
angle.  The  telephoto  lenses  used  for  spotting  would  be  of 
long  equivalent  focus — a  meter  and  more — but  of  handy 
size,  that  is,  not  more  than  50  centimeters  over  all  working 
distance.  The  wide  angle  short  focus  lenses  would  be 
designed  for  low  flying  reconnaissance  or  quick  mapping 
work.  They  would  also  be  demanded  for  peace-time  map- 
ping projects,  where  the  largest  possible  amount  of  territory 
should  be  covered  in  a  single  flight.  Both  types  of  lens 
should  be  pushed  to  the  extreme  in  aperture,  for  short  ex- 
posures and  the  maximum  of  working  days  will  always 
tie  demanded. 
V  A  383 


384        AIRPLANE  PHOTOGRAPHY 

Cameras. — Peace-times  will  give  the  necessary  oppor- 
tunity to  develop  self-contained  and  therefore  simply 
installed  cameras.  They  will  at  the  same  time  be  made 
very  completely  automatic  but  simple  to  operate  in  spite  of 
their  complexity.  Such  cameras  have,  during  the  war,  been 
the  ideal  of  all  aerial  photographers,  but  the  time  has 
been  too  short  since  the  necessary  conditions  have  been 
understood  for  that  lengthy  development  work  and  those 
complete  service  tests  which  are  so  necessary  to  develop 
all  automatic  apparatus.  Several  designs  which  are  now 
being  perfected  may  be  counted  on  to  take  us  a  long  way 
toward  this  ideal. 

On  the  other  hand,  that  military  ideal  which  leaves  the 
camera  operator  the  greatest  possible  freedom  for  other 
activities,  is  apt  to  be  entirely  reversed  in  peace.  The 
camera  operator  can  now  be  required  to  be  an  expert,  who 
will  be  free  to  change  plates  or  niters  and  to  estimate  expos- 
ures, instead  of  giving  his  best  efforts  to  the  problem  of 
defence.  For  him  a  simple  and  reliable  hand-operated  or 
semi-automatic  camera  is  entirely  satisfactory,  and  the  great 
expense  of  complicated  automatic  apparatus  has  no  longer 
its  former  justification. 

Camera  Suspension. — Perhaps  the  most  pleasing  pros- 
pect before  the  aerial  photographer  as  he  turns  from  war  to 
peace  work  is  that  of  having  planes  built  for  and  dedicated 
primarily  to  photography.  Instead  of  his  camera  being 
relegated  to  an  inaccessible  position,  picked  after  the  plane 
design  has  been  officially  "locked;"  instead  of  yielding  first 
place  to  controls,  machine  gun  and  ammunition;  instead  of 
being  jealously  criticised  for  the  space  and  weight  it  takes 
up,  the  camera  can  now  claim  space,  weight,  and  location 
suitable  for  any  likely  aerial  photographic  need.  High  speed 
no  longer  will  be  vital,  and  slower  planes,  permitting  longer 


APPARATUS  385 

exposures  in  inverse  ratio  to  their  speed,  will  be  chosen  for 
photographic  purposes. 

A  development  which  is  sure  to  intrigue  many  investiga- 
tors is  the  gyroscopically  controlled  camera.  This  has  its 
chief  raison  d'etre  in  precision  mapping,  whose  possibilities 
from  the  air  will  undoubtedly  be  intensively  studied  at  once. 
With  the  automatically  leveled  camera  will  come  renewed 
attention  to  indicators  of  time,  altitude,  and  direction,  with 
the  ultimate  goal  of  producing  aerial  negatives  that  show 
upon  their  face  the  exact  printing  and  arranging  directions 
necessary  to  put  together  an  accurate  map. 

Sensitive  Materials. — Manufacturers  of  plates  and  films 
will  direct  efforts  toward  producing  emulsions  cf  good  con- 
trast, high  color  sensitiveness  and  high  effective  speed, 
especially  when  used  in  conjunction  with  the  filters  neces- 
sary for  haze  penetration.  Exposure  data  will  be  accumu- 
lated and  exposure  meters  appropriate  for  aerial  work  will 
be  developed. 

Color  Photography. — Color  photography  from  the  air  by 
any  of  the  screen-plate  or  film-pack  methods  is  probably 
out  of  the  question  because  of  the  long  exposures  required. 
The  screen-plates  are  unsuitable  also  because  of  the  relatively 
large  size  of  their  grain  compared  to  the  detail  of  the  aerial 
photograph.  Ordinary  three-color  photography,  using  three 
separate  negatives,  is  always  subject  on  the  earth's  surface 
to  the  difficulty  that  the  three  negatives  must  be  exposed 
from  the  same  point  of  view,  either  in  succession  or  by  means 
of  some  optical  arrangement  which  is  costly  from  the  stand- 
point of  light.  In  photographing  from  the  air  this  difficulty 
of  securing  a  single  point  of  view  for  the  three  photographs 
is  absent.  Three  matched  cameras,  side  by  side  in  the  fusel- 
age, have  identical  points  of  view  as  far  as  objects  on  the 
earth  below  are  in  question.  Consequently,  three-color 
25 


AIRPLANE  PHOTOGRAPHY 

negatives  are  entirely  possible,  and  indeed  will  be  simple 
to  make  as  soon  as  plates  of  adequate  color  sensitiveness  and 
speed  are  available.  Probably  the  new  Ilford  panchromatic 
plate  has  the  necessary  qualities. 

Night  Photography. — The  searching  eye  of  photography 
was  so  omnipresent  in  the  later  stages  of  the  Great  War 
that  extensive  troop  movements  and  other  preparations  had 
to  be  carried  out  either  in  photographically  impossible 
weather  or  else  at  night.  The  natural  reply  to  the  utilization 
of  the  cover  of  night  is  to  "turn  night  into  day"  by  proper 
artificial  illumination.  At  first  thought  it  might  well  appear 
that  the  task  of  illuminating  a  whole  landscape  adequately 
for  airplane  photography  is  well-nigh  hopeless  by  any  artificial 
means.  On  one  hand  we  have  the  very  short  exposures  alone 
permissible;  on  the  other  the  fact  that  the  intensity  of  day- 
light illumination  is  overwhelmingly  greater  than  those  com- 
mon in  the  most  extravagant  forms  of  artificial  illumination. 

Toward  the  close  of  the  war,  however,  actual  experiments 
made  with  instantaneous  flashes  of  several  million  candle- 
power  showed  that  if  proper  means  were  provided  to  insure 
the  flash  going  off  near  the  ground,  and  if  its  duration  were 
made  no  longer  than  about  -^  second,  interpretable  photo- 
graphs were  obtainable  on  the  fastest  plates.  It  appears, 
therefore,  merely  a  matter  of  manufacturers  perfecting  the 
technique  of  flash  production,  and  of  inventors  providing 
the  launching  and  igniting  devices  to  push  this  kind  of 
photography  to  the  practical  stage.  The  achievement  of 
night  photography  cannot  fail  to  have  an  enormous  effect 
on  future  tactics. 

The  technique  of  night  photography  may  take  either  of 
two  directions.  On  one  hand  we  may  develop  flashes  of  the 
requisite  intensity  to  give  all  their  light  in  y^-  second;  on 
the  other  hand,  it  may  prove  more  feasible  to  use  flashes  of 


APPARATUS  387 

longer  duration  and  to  arrange  for  the  camera  shutter  (of  the 
between-the-lens  type)  to  be  exposed  synchronously  with 
the  middle  of  the  flash.  One  way,  frequently  suggested,  to 
use  these  longer  flashes  would  be  to  trail  the  charge  on  a  long 
wire,  through  which  the  ignition  is  effected  electrically.  This 
is  not  likely  to  be  satisfactory,  however,  for  the  resistance  of  a 
wire  is  so  great  that  when  the  plane  flies  at  any  practical 
height,  the  trailed  flash,  if  it  reaches  near  the  ground,  will 
be  forced  to  a  very  great  distance  behind.  Probably  the 
best  solution  will  involve  accurate  synchronizing  of  the  fuse 
in  the  freely  dropped  sack  of  flash  powder  with  the  exposing 
mechanism  in  the  camera. 


CHAPTER  XXXII 
PICTORIAL  AND  TECHNICAL  USES 

Aside  from  their  element  of  novelty,  aerial  photographs 
have  undouted  qualities  of  beauty  and  utility.  The  "bird's- 
eye  view"  has  always  been  a  favorite  for  revealing  to  the 
best  advantage  the  entire  form  and  location  of  buildings  and 


FIG.  188.— Rheims  Cathedral. 

of  other  large  objects.    Heretofore  such  views  have  usually 
had  to  be  drawn  by  an  imaginative  artist. 

Aerial  oblique  views  possess  the  virtues  both  of  pictures 
and  of  plans.  They  are  destined  to  be  extensively  used  in 
the  study  of  architecture  (Fig.  188).  Cathedrals,  castles, 

388 


USES 


town  halls,  particularly  those  still  in  their  cramped  medieval 
surroundings  where  they  can  never  be  seen  in  their  entirety 
from  the  ground,  come  forth  in  all  their  beautiful  or  quaint 
proportions  from  the  airman's  point  of  vantage.  Stereoscopic 
aerial  views  are  destined  to  occupy  a  valuable  position  also. 
Stereo  prints  of  the  famous  buildings  of  Europe,  taken  from 


FIG.  189. — A  portion  of  Vienna  seen  from  the  air,  during  a  "propaganda  raid." 
Italian  official  photograph. 

the  air,  will  give  to  the  prospective  traveler  or  the  arm-chair 
tourist  a  many  fold  more  accurate  idea  of  their  construction 
than  will  any  number  of  mere  surface  views. 

A  vertical  aerial  photograph  is  most  closely  akin  to  a 
map,  but  has  advantages  over  any  ordinary  surveyor's 
product.  As  a  guide  it  is  infinitely  superior  to  the  best 
draftsman's  diagram,  for  it  provides  a  wealth  of  detail 
whereby  the  traveller  may  definitely  locate  himself.  At  a 


390 


AIRPLANE  PHOTOGRAPHY 


USES 


391 


392 


AIRPLANE   PHOTOGRAPHY 


single  glance  he  notes  the  objects  of  interest  within  his  radius 
of  easy  travel.  The  guide-book  of  the  future  will  therefore 
be  incomplete  without  numerous  aerial  views,  both  vertical 
and  oblique.  As  an  illustration  of  the  peculiar  merit  of  the 
view  from  the  air,  consider  the  photograph  of  Vienna  made 
during  d'Annunzio's  "propaganda"  bombardment.  Or  the 


FIG.  192.— A  sea-side  resort. 

picture  of  the  Rialto  bridge  (Fig.  190).  No  ordinary  photo- 
graph from  land  or  water  suggests  the  central  roadway  and 
no  map  shows  the  beauty  of  its  elevation.  Both  are  shown 
here,  as  well  as  an  intimate  view  of  the  arched  and  pillared 
courtyard  of  the  Fondaco  de'  Tedeschi  to  the  right. 

Airplane  photographs  will  undoubtedly  be  widely  used 
in  certain  fields  of  advertising.     Architects  and  real  estate 


USES 


393 


agents  may  be  expected  to  display  their  wares  by  the  aid 
of  aerial  views.  A  well-planned  country  estate  or  golf  course, 
or  a  suburban  development  (Fig.  191),  can  be  shown  with  a 
completeness,  both  as  to  environment  and  stage  of  progress 
which  no  other  form  of  representation  can  approach.  A  sea- 
side resort  can  now  show  the  extent  and  grouping  of  its 


FIG.  193. — A  bathing  beach  seen  from  the  air. 

natural  and  artificial  amusement  features  in  a  single  picture 
(Fig.  192).  Even  the  extent  of  its  bathing  beach  under  water 
is  revealed  to  the  aerial  photographer  (Fig.  193).  Real 
estate  agents  can  utilize  aerial  photographic  maps  of  cities 
to  great  advantage.  On  these  their  properties  can  be  pointed 
out,  with  the  nature  of  their  surroundings  shown  at  a  glance, 
together  with  their  relation  to  transportation,  schools, 


394 


AIRPLANE  PHOTOGRAPHY 


FIG.  194. — Mt.  Vernon  from  the  air. 


churches,  shopping  districts,  parks,  or  factories.    The  future 
purchaser  of  lots  in  a  distant  boom  town  will   no  longer  be 


USES 


39$ 


satisfied  with  a  map  outlining  the  streets  with  high-sounding 
names.  He  will  demand  an  authentic  aerial  photograph, 
showing  the  actual  number  of  houses  under  construction,  the 
streets,  gutters  and  sidewalks  already  laid,  the  size  and 
planting  of  trees. 


FIG.  195. — A  contrast  in  roofs.    The  Capitol  retains  its  individuality, . while  the  White  House  loses 
all  character  when  seen  from  above. 

The  study  of  landscape  gardening  is  another  field  for 
which  the  aerial  photograph  is  peculiarly  fitted.  A  collection 
of  oblique  pictures  of  the  chateaux  and  palaces  of  Europe 
showing  their  approaches  and  grounds,  or  of  the  historic  es- 
tates of  our  own  South,  (Fig.  194),  will  be  worth  more  to  the 
prospective  designer  of  a  country  estate  than  maps  and 
ground  pictures  can  ever  be.  Closely  allied  to  landscape 


396        AIRPLANE  PHOTOGRAPHY 

gardening  is  city  planning,  for  which  the  aerial  map  will  be 
quite  indispensable.  The  appearance  of  a  city  from  the  air 
may  indeed  become  a  matter  of  pride  to  its  inhabitants,  and 
not  only  the  arrangement  of  streets  and  parks,  but  even  the 
character  of  the  roofs  of  the  buildings,  be  the  subject  of 
study  (Fig.  195). 


FIG.  196. — An  aviation  field  under  construction;  early  stage. 

Engineers  and  constructors  will  depend  more  and  more 
on  preliminary  photographic  surveys  as  a  basis  for  locating 
their  operations.  At  the  later  stages  of  their  work  they  will 
use  aerial  photographs  for  recording  progress.  Periodic 
photographs  of  buildings  in  process  of  construction,  such  as 
are  now  made  from  the  ground,  are  much  more  illustrative 
when  made  from  the  air.  Only  from  above  is  it  possible  to 
obtain  in  a  single  picture  the  progress  of  the  complete  project, 


USES 


397 


such  as  the  construction  of  an  aviation  field  (Figs.  196  and 
197)  or  of  a  shipyard.  The  building  of  large  structures^ — 
bridges,  hotels,  ships  on  the  stocks — particularly  demands 
aerial  views  if  the  foreground  is  not  to  eclipse  the  center  of 
real  interest. 


FIG.  197. — An  aviation  field  under  construction;  later  stage. 

News  events  will  soon  call  for  an  aerial  photographic 
service.  Already  we  are  seeing  newspapers  and  magazines 
featuring  aerial  photographs  of  the  entry  into  conquered 
cities  and  the  parades  of  returning  fleets.  Accidents,  fires, 
floods  and  wrecks,  of  either  local  or  national  interest,  can 
best  be  represented  by  this  newest  form  of  photography. 

The  photographing  of  wrecks,  fires  and  floods  suggests 
the  importance  of  aerial  views  to  insurance  underwriters, 


398        AIRPLANE  PHOTOGRAPHY 


USES 


399 


who  require  the  most  minute  information  on  the  character- 
istics of  buildings  in  every  neighborhood,  and  on  the  extent 
and  nature  of  damage  done.  Marine  insurance  companies 
might  with  profit  use  the  airplane  camera  to  help  estimate 


FIG.  199. — Waves  set  up  by  a  ship — of  interest  to  the  naval  architect. 

the  chances   of    salvage  of    a  stranded  ship  or    a    vessel 
foundered    in   shallow   waters    (Fig.    181). 

Numerous  scientific  uses  for  aerial  views  seem  likely. 
Prominent  among  these  is  their  use  in  geology,  for  the  study 
of  the  various  forms  of  earth  sculpture.  Pictures  from  the 
air  of  extinct  volcanoes  will  give  information  as  to  their 


400        AIRPLANE  PHOTOGRAPHY 

configuration  that  would  otherwise  require  months  of  pains- 
taking survey  to  obtain.  Aerial  photographs  of  active  vol- 
canoes (Fig.  198),  showing  the  results  of  a  succession  of  out- 
bursts— one  obliterating  the  other — would  prove  of  the  great- 
est value,  especially  when  studied  in  conjunction  with  other 
scientific  data,  the  whole  making  a  record  unobtainable  by 
any  other  means. 

In  earthquake  regions — notably  Southern  Italy  and 
Japan — the  changing  coast  lines,  shallows  and  safe  harbors, 
could  be  promptly  ascertained  after  the  subsidence  of  each 
fresh  shock,  with  a  consequent  keeping  open  of  trade  routes 
and  often  the  saving  of  life.  River  courses,  glacier  forma- 
tions, canons,  and  all  the  larger  natural  formations  which 
man  usually  sees  only  in  minute  sections,  and  which  he 
must  build  up  in  his  mind's  eye  or  by  models,  are  today 
quickly  and  accurately  recorded  by  the  camera  in  the  air. 
Such  formations  as  coral  reefs,  whose  configurations  can 
now  be  accurately  learned  only  by  laborious  surveys  of  a 
limited  number,  could  be  studied  in  quantity  and  with  here- 
tofore unknown  satisfaction  as  the  result  of  a  single  expedi- 
tion with  a  ship-carrying  seaplane  and  aerial  camera. 

Another  scientific  field — probably  one  of  many  similar 
ones — lies  in  the  study  of  the  waves  set  up  by  ships  (Fig.  199). 
These  are  of  extreme  importance  in  the  realm  of  naval 
architecture,  but  before  the  day  of  the  airplane  could  never 
be  easily  studied  in  full  scale. 


CHAPTER  XXXIII 
EXPLORATION  AND  MAPPING 

Aerial  photographic  mapping  in  war-time  has  been 
almost  entirely  confined  to  inserting  new  details  in  old 
maps.  For  such  work  some  distortion  or  a  lack  of  complete 
information  on  altitude  and  directions  is  not  a  serious  mat- 
ter, because  the  known  permanent  outlines  serve  as  a 
basis.  Furthermore,  in  so  far  as  outline  maps  are  concerned, 
as  distinguished  from  pictorial  maps,  these  have  been  drawn 
on  the  ordinary  scales,  and  with  the  ordinary  conventions  of 
engineering  map  practice. 

Aerial  photography  may  be  used  in  the  future  in  prac- 
tically the  same  way,  as  an  aid  to  the  quick  recording  of 
those  minute  details  which  would  ordinarily  consume  an 
enormous  amount  of  labor  to  survey  directly.  The  region 
shown  in  Fig.  200  affords  a  good  illustration.  A  discouraging 
amount  of  time  and  effort  would  be  required  to  map  this 
section  of  Virginia  by  the  usual  methods,  while  the  smallest 
curve  of  creek  and  shore  is  instantly  and  completely  recorded 
on  a  single  photographic  plate.  But  there  are  other  possibili- 
ties, diverging  from  this  application  both  toward  greater 
and  lesser  requirements  for  precision. 

Pictorial  maps,  in  which  the  actual  photographs  figure, 
promise  to  be  an  essential  part  of  the  airman's  equipment, 
whether  he  be  pilot  or  passenger,  mail  carrier  or  sports- 
man. Without  any  pretention  to  detailed  accuracy  of 
location,  these  maps  will  show,  in  strip  or  mosaic  form,  the 
general  appearance  of  the  country  to  be  traversed,  with 
particular  reference  to  good  landing  fields  and  other  points 
of  interest  to  the  aviator.  Vertical  pictorial  maps  may  be 

26 


402        AIRPLANE  PHOTOGRAPHY 

supplemented  by  obliques  giving  the  view  ahead,  whereby 
the  pilot  may  direct  his  ship.  Thus  the  Washington  monu- 
ment as  seen  by  the  pilot  from  Baltimore  is  a  truer  guide 
than  is  the  country  beneath  him.  The  crossing  of  mountain 
ranges  is  another  case  where  the  oblique  picture  will  be  more 
useful  than  the  vertical  (Fig.  201). 


FIG.  200. — An  aerial  photographic  survey  of  ground  difficult  to  cover  by  ordinary  surveying  methods. 

Contrasted  with  the  merely  pictorial  maps  will  be  pre- 
cision surveys.  Whether  it  will  prove  practical  to  make 
these  entirely  from  the  air  is  still  an  open  question.  It  is 
to  be  assumed  that  cameras  can  be  constructed  with  lenses 
having  negligible  distortion  of  field,  with  between-the-lens 
shutters  to  obviate  the  distortions  due  to  the  focal-plane 
type,  with  auxiliary  devices  for  indicating  compass  direction, 


EXPLORATION  AND  MAPPING   403 


404        AIRPLANE  PHOTOGRAPHY 

altitude,  and  inclination,  or  with  gyroscopic  mounting  so 
that  an  inclination  indicator  is  unnecessary.  The  applica- 
tion of  aerial  photography  to  precision  mapping  will  depend 
upon  the  perfection  which  such  cameras  attain,  as  estimated 
by  the  permissible  errors  in  this  form  of  mapping.  Entire 
dependence  on  photography,  as  in  uncharted  regions,  is 
likely  to  be  worked  up  to  slowly,  beginning  with  a  stage 
of  rather  complete  triangulation  of  natural  or  artificial 
points — say  three  in  each  constituent  picture — then  through 
several  stages  each  successively  employing  fewer  and  fewer 
well  determined  points.  The  photographic  mapping  of 
some  of  our  Western  States  will  be  greatly  facilitated 
by  the  100-yard  squares  into  which  the  land  is  divided 
and  already  marked  in  a  manner  which  shows  clearly  in 
aerial  photographs. 

A  theoretical  possibility  is  the  plotting  of  contours  from 
stereo-aerial  pictures.  Given  two  elements  of  a  stereo- 
scopic pair,  taken  from  points  whose  separation  is  known, 
the  position  of  any  point  in  space  shown  in  the  stereoscopic 
view  can  be  determined  by  the  use  of  the  stereocomparator. 
This  is  an  instrument  already  employed  in  mountain  photo- 
surveying,  which  consists  essentially  of  a  compound  stereo- 
scope in  whose  eye-pieces  are  two  points  movable  at  will 
so  that  the  relief  image  formed  by  their  fusion  can  be  made 
to  coincide  with  any  chosen  part  of  the  landscape. 
The  chief  difficulty  in  the  application  of  this  idea  to  aerial 
work  is  to  fix  the  base  line.  This  problem  may  be  met  in 
some  cases  by  using  stereo  obliques,  and  getting  the  base 
line  by  simultaneously  made  vertical  photographs  of  well 
surveyed  territory  beneath.  Possibly  also  methods  can  be 
developed  by  which  photographs  from  two  or  more  known 
altitudes  may  furnish  the  requisite  data. 

City  mapping  is  a  field  for  which  aerial  photography  is 


EXPLORATION   AND   MAPPING   405 


406         AIRPLANE  PHOTOGRAPHY 


FIG.  208. — Mosaic  map  of  the  City  of  Washington.    White  rectangle   shows    portion   included   in 

next  figure. 

peculiarly  fitted  (Fig.  202).      A  complete  map  of  a  large 
city  is  a  labor  of  years.     In  fact,  a  modern  city  is  always 


EXPLORATION  AND  MAPPING   407 

wm 


FIG.  204. — Portion  of  Washington  mosaic,  full  size. 

dangerously  near  to  growing  faster  than  its  maps.  An 
aerial  map,  on  the  contrary,  can  be  produced  in  a  few  hours. 
Paris  was  mapped  with  800  plates  in  less  than  a  day's  actual 
flying.  Washington  was  completely  mapped  in  2^  hours, 


408        AIRPLANE  PHOTOGRAPHY 

with  less  than  200  exposures.  The  entire  map  is  shown,  on  a 
greatly  reduced  scale,  in  Fig.  203,  while  Pig.  204  shows  a 
small  portion  of  it  in  full  size,  from  which  can  be  obtained  an 
idea  of  the  dimensions  of  the  original.  These  maps,  while 
not  accurate  enough  for  the  recording  of  deeds  and  mort- 
gages, yet  serve  the  majority  of  needs.  There  is  indeed  no 
reason  why  with  long  focus  cameras,  given  several  accurately 
marked  points,  the  photographic  map  of  a  piece  of  real 
estate  should  not  be  made  with  all  the  accuracy  needed,  still 
leaving  the  whole  process  of  partial  surveying  helped  out  by 
photography  an  enormously  simpler  one  than  the  usual  method . 
Rougher  types  of  surveying,  in  open  country,  offer  a 
most  promising  opportunity.  Railway  surveys,  showing 
the  character  of  the  country:  passes  through  mountain 
ranges :  the  available  timber  and  other  materials  of  construc- 
tion. Canal  routes,  with  the  available  sources  of  water  supply, 
and  the  best  choice  of  course  to  avoid  deep  cuttings  and 
aqueducts.  Irrigation  projects,  with  the  natural  lakes, 
river  courses  and  valleys,  which  may  be  dammed  to  form 
storage  basins.  Coast,  river  and  harbor  surveys  are  possible 
by  aerial  means  with  a  promptness  and  frequency  which 
should  entirely  revolutionize  the  making  of  maps  of  water- 
ways. Shifts  in  channels  and  shallows,  even  of  considerable 
depth,  stand  out  prominently  in  the  aerial  photograph. 
The  actual  bottom,  if  not  more  than  three  or  four  meters 
down — as  in  a  bathing  beach — shows  in  the  aerial  photo- 
graph (Fig.  193),  while  the  varying  surface  tints  caused  by 
light  reflected  from  the  bottom  at  far  greater  depths  are 
readily  differentiated  by  the  camera  from  the  air.  An 
instantaneous  photograph  will  thus  perform  the  work  now 
done  by  a  week's  soundings.  Fig.  205,  taken  near  Langley 
Field,  shows  how  the  aerial  photograph  may  be  used  to  chart 
natural  channels,  while  Fig.  206  shows  the  dredged  chan- 


EXPLORATION  AND  MAPPING    409 


FIG.  205.— Shallows  and  channels  revealed 


410        AIRPLANE  PHOTOGRAPHY 


EXPLORATION  AND  MAPPING  411 


412        AIRPLANE  PHOTOGRAPHY 


EXPLORATION  AND   MAPPING    413 

pels  of  the  port  of  Venice.  Navigation  of  such  a  river  as  the 
Mississippi  with  its  shifting  bars  may  come  to  be  guided  by 
monthly  or  even  weekly  aerial  photo  maps. 

Among  other  uses  for  aerial  photography  will  be  the  loca- 
tion of  timber.  As  one  illustration,  may  be  taken  the  dis- 
covery of  mahogany  trees.  Their  foliage  at  certain  times  of 
the  year  is  of  characteristic  color.  This  may  be  recorded 
on  color  sensitive  plates  with  a  scientifically  chosen  filter, 
and  the  cutting  expedition  sent  out  with  the  photograph  as 
a  guide.  In  this  as  in. other  cases  where  rough  or  unexplored 
country  is  to  be  covered,  it  is  a  question  whether  the  air- 
plane will  after  all  be  the  most  feasible  craft,  on  account  of 
its  necessarily  rapid  rate  of  travel,  and  its  need  for  known 
landing  fields.  The  dirigible  of  large  cruising  radius,  which 
can  seek  its  landing  field  at  leisure,  is  probably  indicated  for 
this  kind  of  work.  It  may  indeed,  as  already  hinted,  prove 
to  be  the  chief  photographic  aircraft  of  the  future. 

Archaeological  surveys  offer  a  fascinating  opportunity 
for  airplane  or  dirigible  balloon  photography  to  render 
scientific  service.  Buried  in  desert  sands  or  overgrown  with 
tropical  vegetation  the  ancient  cities  of  Asia  Minor,  of 
Burma,  and  of  Yucatan  evade  discovery,  and  even  when 
found  remain  unmapped  for  decades.  Discovery  and  map- 
ing  can  now  go-hand-in-hand.  The  topography  of  barbaric 
or  colonial  towns  and  villages,  whose  importance  could 
not  warrant  elaborate  surveys,  but  which  should  neverthe- 
less be  a  matter  of  record,  will  be  quickly  and  easily  plotted 
by  photography  (Fig.  207).  To  this  day  who  knows  how 
the  streets  run  in  Timbuctu,  and  how,  save  from  the  air,  can 
we  ever  map  the  teeming  cities  of  China?  He  who  would  fol- 
low in  the  footsteps  of  Haroun-al-Raschid  can  even  now  ex 
plore  the  by-ways  of  Bagdad  by  the  aid  of  the  Royal  Air 
Force  photographic  map! 


INDEX 


Aberrations,  lens,  47,  54 
Aberration,  spherical,  47,  54 

chromatic,  48,  54 
Acetylene  light,  284 
Advertizing,  use  of  aerial  photography 

in,  392 

Aero  1  and  Aero  2  filters,  241 
Airplane,  as  camera  platform,  20 

types  of,  24 
Air  speed,  27,  308 

indicator,  33,  34 
Alcohol,  use  of  in  plate  drying,  275 

use  of  in  print  drying,  286 
Altimeter,  33,  174 

reading  recorded  on  film,  135,  171 
Altitudes  of  flight  for  photography,  61, 

224 

Anastigmatic  type  of  lens,  47 
Aperture,  lens,  39,  44,  56,  57 
Archaeological  surveying,  413 
Astigmatism,  49,  50,  54 
Auxiliaries,  camera,  163 

installation  of,  214 

Bagley  camera,  286,  314 

Balance,  of  camera,  96 
of  plane,  157,  208,  217 

Balloons,  15,  16,  18 

Banking,  27,  324 

Batteries,  storage,  150,  156 

Bay,  camera,  120,  210 

Bellows,  camera,  65,  95 

Biplane,  24 

Blast  marks  in  front  of  guns,  363 

Bleaching  out  print  to  leave  interpreta- 
tion marks,  367 

Boat,  flying,  25,  374 


Bowden  wire,  103,  106,  108,  111,  114, 

116,  118,  126,  129,  136,  163 
Brightness,  range  of,  221,  225 
Burchell  photographic  slide  rule,  303, 

339 

Camera,  airplane,  39,  384 

automatic,  18, 43, 90, 116, 124, 125,311 

B.  M.,  120,  203 

Bagley,  286,  314 

Brock  automatic  plate,  126 

C  type,  43,  87,  103,  109 

classification  of,  43 

deMaria,  86,  89,  103 

deRam,  82,   93,   121,   129,   157,   205, 

214,  326 

E  type,  43,  87,  103,  109 
elements  of,  42 
film,  see  Film  cameras 
Folmer  automatic  plate,  126 
hand  held,  95,  321,  369 

English,  99 

German,  99 

U.  S.  Air  Service,  100 
lea,  103 

Lamperti  (Italian),  112,  211 
L.  B.,  120 

long  focus,  103,  301,  324 
L  type,  43,  82,  94,  102,  117,  162,  210 
M.,  106,  203 
non-automatic,  43,  102 
Piserini  and  Mondini,  111 
semi-automatic,  43,  116,  149 
stereoscopic,  341 
Camouflage,  filters  for  the  detection  of, 

225,  243 

stereoscopic  views  and,  329,  358 
415 


416 


INDEX 


Ceiling  of  plane,  27,  130 

Channels,  detection  of  by  photography, 

408 

Chemicals,  photographic,  257 
Chlorhydrochinon  developer,  261 
Chromatism,  lateral,  49,  54 
City  planning,  use  of  aerial  photography 

in,  396 

Clock-work  for  driving  cameras,  149, 155 
Clouds,  224,  242 
Collimator,  66 

Color,  coefficient  or  index  of  negative, 
259 

filters,  see  filters 

photography  385 

sensitive  plates,  15,  174,  233,  237 

sensitiveness  of  film,  131 

sensitizing,  methods  of,  235 
Coma,  47,  48 

Communication,  means  of  on  plane,  296 
Compass,  33,  173 

reading  recorded  on  film,  135,  171 
Cone,  camera  lens,  42,  114 

interchangeable,  108,  120 
Construction  operations,  aerial  photo- 
graphic records  of,  396 
Contact,  imperfect  in  printing,  279,  284 

prints,  45,  279 

Contacts,  electric,  on  plane,  163 
Contours  by  stereo  aerial  photography, 

404 
Control,  distance,  of  camera,  110,  163 

speed,  of  camera,  136,  144,  157 
Controls,  duplicate,  19,  25,  195,  209,  214 

of  plane,  21,  26 
Convergence  point,  337 
Cord  for  adjusting  shutter  aperture,  82 
Core  rack  development,  271 
Contrast,     in     brightness    on     earth's 
surface,  221 

in  photographic  emulsions,   15,   230, 
236,  258 


Counter,  exposure,  on  magazine,  88 

on  release,  164 

Covering  power  of  lens,  44,  49,  50,  58 
Crabbing,  27,  308 
Cradles,  camera,  195 
Cross  wires,  23 

insertion  of  camera  through,  42,210 
Curtain,  auxiliary  shutter,  80,  84,  106 

speed  of  travel  of  shutter,  74 

uniformity  of,  76,  82,  84,  86,  315 
Cylinders,    relation   between    vibration 

and  number  of,  185 

Dark  slides,  double,  87,  99 
Daylight,  intensity  of,  222 
Definition,  lens,  44 
Density,  of  air,  effect   of   on   propeller, 

154 

of  photographic  image,  228 
Developers,  for  plates  and  films,  257, 260 

for  papers,  262 

Developing  machines,  film,  133,  273 
Ansco,  273 
Brock,  274 
Eastman,  274 
G.E.M.,  273 

Development,  core  rack  method  of,  271 
factor,  228 
film,  272 

methods  of,  267,  269 
of  prints,  286 
speed  of,  236,  267 
tank,  270 
time,  269 
DH4  plane,  210,  217,  296 

photographic,  213 
DH9  plane,  296 
Diaframs,    to   equalize   illumination   of 

plate,  78 
lens,  48,  58 

Dilution  coefficient  of  developer,  258 
Dirigibles,  15,  16,  413 


INDEX 


417 


Distortion,  absent  with  between-the-lens 

shutter,  74 
barrel,  51 
due  to  camera  tilting,  206,  286,  305, 

315 

in  aerial  maps,  317 
lens,  39,  44,  51,  54,  56,  62 
pin-cushion,  51 

produced  by  film  shrinkage,  237 
produced  by  focal  plane  shutter,  74 
produced  by  glass  plate  in  front  of 

film,  131 

with  wide  angle  lenses,  63     ( 
"Dodging"  in  printing,  279,  315 
Doors    in    plane   for   camera   to   work 

through,  214 
Drying  of  films,  267,  276 
of  plates,  267,  275 
of  prints,  286 

Earth,  appearance  of  from  plane,  30 

Eastman  apron  film  developing  machine, 

274 

twin   reel    film   developing    machine, 
274 

Efficiency,  propeller,  155 
shutter,  70,  72,  76 

English    aerial    photographic    practice, 
45,  46,  283,  291,  340 

EK  filters,  241 

Electric,    drive   for    cameras,  116,  119, 

123,  145,  149 

generator  operated  by  motor,  146 
motor,  characteristics  of,  151,  156 
motor,  service,  163 

Elevation  possible  to  detect  in  stereo- 
scopic views,  340 

Emulsions,    photographic,    characteris- 
tics of,  227 

Enlarging,  45,  279,  283 
camera,  283 
27 


Enlarging  versus  contact  printing,  59 
Exhaustion  of 'developer,  259 
Exploration,  use  of  aerial  photography 

in,  401 
Exposure,  data  charts,  250 

distance   between  for   mosaic  maps, 
307 

distance  between  for  stereos,  336 

estimation  of,  248 

limitations  to,  247 

meters,  251 

meter,  Wynne,  251 

of  aerial  negatives,  247 

relation  between  motion  of  plane  and, 
68,  185 

under,  period,  230 

Field,  angular,  of  lens,  57,  302 

flatness  of  lens,  56 
Field  laboratory,  mobile,  268 
Film  cameras,  43,  130,  134 

Brock,  138 

Duchatellier,  131,  136 

F  type,  134,  171 

G.E.M.,  138 

German,  64,  76,  139,  317 

K  type,  142,  203,  214 
Film,  celluloid,  130 

backed,  133 

changing  in  the  air,  137,  144 
color  sensitiveness  of,  237 
cut,  275,  278 
development  of,  130,  272 
drying,  131,  276 
means  for  holding  flat,  130,  131 
relative  performance  of  compared  to 

plates,  237 

satisfactory  kinds  for  aerial  work,  238 
shrinkage,  237 
Filters,  15,  58,  106,  174,  224,  233,  239, 

241,  243 
effects  secured  by  use  of,  241 


418 


INDEX 


Filters,  gelatin,  244 
glass,  245 

holders  for,  67,  106,  224 
ratio,  240 
Fixing  bath,  262 
Flaps,  auxiliary  to  shutter,  80,  84,  85, 

98,  99,  101,  119 
to  protect  lens,  214 
Flash  lights,  18,  386 
Flying  boat,  374 

Focal  length,  relationship  of  lens  char- 
acteristics to,  55,  57 
requirements  for,  39,  42,  58,  61,  103 
Focal  plane,  50 
shutter,  70,  71 
distortion  by,  74 
performance  of,  86 
types  and  representative,  80 
Focus,  depth  of,  44 

effect  of  temperature  on,  41,  65 
fixed,  in  aerial  cameras,  40 
ground  and  air,  65 
Focussing,  automatic,  284 

by  parallax,  65,  66 
Fog,  atmospheric,  224 

photographic,  237,  258 
French  aerial  photographic  practice,  45, 

46,  283 

Friction  disc  speed  control,  136,  159,  100 
Fuselage,  21,  87     : 

shape  and  size  depending  on  type  of 

engine,  23 
Future  of  aerial  photography,  383 

German   aerial   photographic   practice, 

63,  103,  209 

Glass,  optical,  used  in  lenses,  44 
Gloves,  handling  apparatus  through,  41, 

89 

Governor  for  camera  speed,  159 
Graphite,  use  of  to  prevent  static  dis- 
charge, 134 


Gravity,  action  of  in  aerial  cameras,  41, 

112,  115,  119 
center    of,     should     not     change    in 

cameras,  125,  207 
change  of  center  of,  in  magazines,  92, 

112 

depended  on  in  magazines,  88 
handles  best  at  center  of,  96 
pseudo,  in  moving  vehicle,  188 
support  at  center  of,  182,  203 
Ground  speed,  27,  307 

indicator,  ^511 

Guide-books,  illustration  of,  15,  392 
Guides  for  inserting  magazines,  41 
Gyroscope,  189 
Gyroscopically   controlled  instruments, 

29,  174,  192,  312 
Gyroscopic  erector,  188 

mounting  of  camera,  187 
stabilizer,  Gray,  190 

Hardener,  acid,  262 

Hand  operation  of  deRam  camera,  121, 

125,  129 

Haze,  15,  30,  223,  233,  239 
Head  resistance  and  weight,  equivalent, 

156 
Heat,  effect  of  on  plate  sensitiveness, 

175,  232 

Heater,  electric,  in  camera,  142, 174,232 
Horizon,    photography    of    to    indicate 
inclination,  174 

position  of,  30 
Hurter     and     Drifneld     sensitometric 

curve,  227 

Illumination  of  field  by  lens,  50,  54,  56, 

57 
Image  of  point  source,  size  of,  49,  54,  56 

size  of  in  relation  to  focal  length,  59 
Incidence,  angle  of  assumed  by  plane  at 

high  altitudes,  206 


INDEX 


419 


Inclinometer,  33,  35,  171,  173 
Indicators,  distance,  163,  295 
Inertia,  229,  257 
Installation,  camera,  24,  208 
Instructions,  operating,  to  be  placed  on 

apparatus,  42 
Instruments,  airplane,  30 
Instrument  board,  30 

photographing,  170 

Intensification  of  aerial  negatives,  262 
Intensity  of  daylight,  222  t 
Interpretation  of  aerial  photographs,  17, 

40,  351 

Interval    between   exposures,    40,    158, 
305,  307 

for  stereoscopic  pictures,  334,  338 

methods  of  regulating,  124 
Isochromatic  plates,  233 
Italian  photographic  practice,  122,  377 

Jamming  of  cameras  in  operation,  119, 
120 

Ki  and  K2  niters,  239 

Keeping  power  of  developer,  259 

Kites,  15,  16,  18 

Laboratory,  mobile  photographic,  269 
Landscape  gardening,  use  of  aerial  pho- 
tography in,  395 
Latitude  of  plates,  229 
Lens,  42,  39,  44,  383 

aperture,  39 

characteristics,  46 

mounts,  65 

suitable  for  aerial  photography,  62 

symmetrical,  52 

telephoto,  61 

testing  and  tolerances,  52 

unsymmetrical,  52 

wide  angle,  62 
Levels,  spirit,  on  camera,  95 


Light,  distribution  of  in  aerial  view,  221 
trail  method  of  testing  camera  mount- 
ings, 183 
Loop,  centrifugal  force  in,  29 

Machine  gun  ring  as  camera  mount,  321 
Magazines,  87 
bag,  88,  101 
Bellieni,  92 
Chassel,  92 
deMaria,  89,  98 
Ernemann,  89 
Folmer,  90 
Fournieux,  92 
Jacquelin,  92 
Piserini  and  Mondini,  90 
Ruttan,  92 
Magazine  racks,  94 

installation  of,  217 
Mapping,  64,  135,  185,  186,  304,  401 

precision,  317,  404 
Maps,  mosaic,  17,  39,  64,  314 

sketch,  314 

Marking  of  negatives^  278 
Metol-hydrochinon  developer,  261 
Mirrors  for  oblique  photography,  324, 

326 

Monoplane,  24 
Motions  of  camera,  179 
Motive  power  for  aerial  cameras,  145 
Mounting,  camera,  102,  193,  179,  183 

384 

bell-crank,  120,  198,  203 
Brock,  139,  207 
center  of  gravity,  205 
floor,  195 

G.  E.  M.,  138,  207 
Italian,  207 
outboard,  194 
parallel  motion,  198 
pendular,  185 
tennis  ball,  196 


420 


INDEX 


Mounting,  camera,  turret,  312 

of  prints,  316 

of  stereograms,  346 

Movement,  of  film  during  exposure,  75, 
142 

of  image,  permissible,  68 
Moving  pictures  from  plane,  350 
Mud  splashing  on  camera,  214 

Naval  aerial  photography,  368 
Negative  lens  sight,  168,  299,  305 
Night  photography,  386 
Numbering  devices  in  cameras,  169 

Oblique  views,  39,  320 

angles  at  which  taken,  40,  321 
exposures  for,  69 
filters  for,  242 
use  of  hand  cameras  for,  95 
Observer,    function    of   in    aerial    pho- 
tography, 291,  295,  304 
Oil  spray  from  motor,  214 
Opacity,  228 
Opening  for  camera,  211 
Opposite   directions,    shutter   to   move 

alternately  in,  76,  139 
Orthochromatic  plates,  233 
Overlaps,  for  mapping,  307 
for  stereoscopic  views,  40 
on  a  turn,  64,  139 

Panchromatic  emulsions,  233 

plates,  49,  238 
Panoramic  views,  321 
Parallax  method  of  focussing,  65,  66 
Parallel,  flying  in,  308 
Photographic  planes,  special,  213 
Pilot,  function  of,  in  aerial  photography, 

291,  295 

Pin-points,  39,  291 
Pistol  grip  for  hand  cameras,  95,  96 
Plate  holders,  87 


Plates,  bathed,  235 

behavior  of  compared  to  film,  237 

color  sensitive,  233 

iso  and  ortho-chromatic,  233 

panchromatic,  233 

satisfactory  kinds  for  aerial  work,  238 

self  screening,  245 

shape  of,  63 

size  of,  43,  62 

Plumb  line,  behavior  in  banking  plane,  28 
Ply- wood  veneer  construction,  23,  211 
Positype  paper,  238 
Potassium  carbonate  for  drying  plates, 

276 

Power  required  to  drive  cameras,  125 
Pressure,  of  shutter  curtain,  1?1 

plate,  for  holding  film  flat,  131,  141 
Printing,  279 

contact,  279 

machines,  279 

media,  252 
Prints,  paper,  253 

development  of,  286 
Prisms  for  oblique  photography,   324, 

326 
Propeller,  characteristics,  152 

constant  speed,  129,  159 

drive  for  cameras,  102,  116,  119,  135, 
136,  144,  157,  158 

position  of,  120 

variable  speed,  159 

Pump,  for  producing  suction  on  film,  132 
Punch  marks  on  film,  144,  272 
Pyro  developer,  261 

Racks,  negative,  271 

Real  estate,  aerial  mapping  of,  393 

Rectifying,  286,  305 

camera,  314 

Reduction  of  aerial  negatives,  262 
Release,  shutter,  96,  99 

duplicate,  for  pilot,  164 


INDEX 


421 


Release,  shutter,  time  controlled,  124 
Relief,  criterion  for  correct,  335,  344 
exaggerated,  337 
impression  of,  produced  by  motion, 

349 

Resolving  power  of  plates,  59,  235,  260 
Richard  stereo  printing  frame,  283,  348 
Rinsing  of  plates,  259 
Rubber,  sponge,  use  of  in  camera  mount- 
ings, 195,  203 

Safe  lights,  photographic,  269 

Safety  catch  on  camera  mounting,  203 

device  on  camera  driving  mechanism, 

162 
Salvaging  of  ships,  aerial  photography 

and,  399 

Seaplane,  25,  374 
Self  screening  plates,  245 
Semaphore  signalling  in  plane,  297 
Semperfocal  enlarging  camera,  284 
Sensitized  materials,  requirements  for, 

225 
Sensitometry,  227 

of  papers,  253 
Shadows,  compass  directions  from,  356 

proper    direction    for,    in    examining 

prints,  352 

Shaft,  flexible,  119,  123,  142,  161 
Sheaths,  plate,  87,  88,  93,  126,  170 
Shrinkage,  film,  237,  317 

paper,  285,  315,  317 
Shutter,  42,  68 

between-the-lens,    58,    70,    112,    115, 
316,  387 

efficiency,  70,  72,  76 

focal  plane,  70,  71,  73 

focal  plane,  double  for  stereo  work, 
341,  345 

focal    plane,    moving    alternately    in 
opposite  directions,  76 

focal  plane,  types  of,  80 


Shutter,  Folmer,  80 

lea,  81 

Klopcic,  78,  84,  98 

release,  96,  99,  124 

speed,  39,  40,  58,  70,  249 

testing,  76 

testing  apparatus,  77 
Sights,  164,  166,  296,  301,  327 

adjustable  for  angle  of  incidence  of 
plane,  169 

attached  to  plane,  167 

negative  lens,  168,  299,  305 

rectangular,  98,  167,  373 

stereoscopic,  338 

to  indicate  size  of  field,  166,  373 

tube,  101,  166 
Single-seaters,  carrying  cameras  in,  114, 

211 

Slide  rule,  photographic,  303,  339 
Solenoid,  151,  163 
Sound,  not  to  be  used  for  indication  in 

plane,  164 

Spacing  of  pictures  in  film  camera,  144 
Speaking  tubes  on  plane,  296 
Speed,  of  development,  236,  267 

of  plates,  228,  236 
criteria  of,  230 
effect  of  temperature  on,  175,  232 

variable,  control  of  camera,  144,  151 
Spotting,  64,  125,  291 
Spring  motors,  149,  155 
Springs,  use  of  in  aerial  cameras,  41 

in  magazines,  88 
Stabilized  camera,  95,  187 
Static  electric  charges  on  film,  131,  133 
Stereocomparator,  404 
Stereo-oblique  views,  115,  321,  343 
Stereoscopes,  331 
Stereo  printing,  283 
Stereoscopic  cameras,  341,  344 

effect,  absence  of  at  flying  heights,  30, 
334 


422 


INDEX 


Stereoscopic  photography,  329 
mounting,  346 

pictures,    fusion    of    without    instru- 
ments, 330,  343 
sights,  338 
views,  39,  40,  64 

uses  for,  348 
vision,  principles  of,  329 
Stop-watch  attached  to  shutter  release, 

310 
Strap,  on  hand  camera,  to  go  around 

observer's  neck,  99 
on  plate  magazine,  87 
to  go  over  hand,  on  hand  camera,  100 
Stream  lines,  26 

lined  hood,  214 
Suction  for  holding  film  flat,  131,  132, 

142 

advantages  of  continuous  and  inter- 
mittent, 132 
Surveying  by  aerial  photography,  401 

Tank  development,  270 

Tearing  edges  of  prints,  317 

Telephones  on  planes,  296 

Telephoto  lens,  61 

Temperature,    coefficient    of    develop- 
ment, 258 

effect  of  on  focus,  41 
effect  of  on  mechanical  functioning, 

41,  102,  125 

•  effect  of  on  plate  speed,  232 
limits  of  development,  258 

Test  chart,  lens,  52 

Threshold  value,  230 

Thrust,  propeller,  152 

Timber,   location  of  by  aerial  photog- 
raphy, 413 

Tone  rendering,  correct,  226,  230,  239 

Touch,    sense    of,    not    dependable    in 
plane,  41,  125,  164 


Trailer,  photographic  truck  and,  268 
Transmission  of  power  to  camera,  161 
Transparencies,  252,  330 
Transparency,  227 
Trays,  camera,  195 
Tri-color  ratio,  235 
Triplane,  24 

Tuning  fork,  used  in  shutter  tester,  79 
Turbine,  wind,  for  driving  camera,  116, 
127,  144,  147,  158 

Uniformity   of  curtain   speed   in   focal 

plane  shutter,  76,  82,  84,  86,  315 
Unit  system  of  camera  construction,  42 
Uprights,  camera,  in  plane,  209 
Uses  for  aerial  photography,  388 

Velocity  constant,  258 

Veneer  construction,  23,  209,  211 

Venturi  tube,  132,  142,  144 

Vibration,  16,  18,  26,  40,  41,  58,  102,  179 

Volcanoes,  photography  of,  15,  399 

Water  for  mixing  chemicals,  262 
Watkins  factor,  258 

Weight    and   head   resistance,    equiva- 
lence, 156 

of  film  compared  to  plates,  101,  130 

of  deRam  camera,  123 

of  hand  cameras,  96 

of  K  type  camera,  144 

of  K  film  roll,  144 

of  L  type  camera,  117 

of  M  type  camera,  109 

of  storage  batteries,  157 
Wind,  flying  against,  68,  308 

motor,  146 

Windows  in  side  of  plane,  214,  328 
Wire,  barbed,  appearance  of  in  aerial 

photograph,  360 


^C'D  LO 



WAV  2  4  I960 


LD  2lA-50m-8  '57 
(C8481S10)476B 


.  General  Li 
University  of 

Berkeley 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


